Write some info in README.md, design titlebar, make downloader and very basic instance checker

lib/checker.js

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+const path = require("node:path");
+const fs = require("node:fs");
+
+module.exports = {
+	"check_install": function (directory) {
+		return new Promise(function (resolve, reject) {
+			resolve(fs.existsSync(path.join(directory, "versions")));
+		});
+	}
+};

lib/downloader.js

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+const path = require("node:path");
+const fs = require("node:fs");
+const fsPromises = require("node:fs/promises");
+
+const httpsWrapper = require("./util/https.js");
+const utilities = require("./util/functions.js");
+
+let maxConcurrentDownloads = 16;
+let downloadQueue = [];
+let totalDownloads = 0;
+let isDownloading = false;
+let ongoingDownloads = 0;
+
+function waitUntilDownloadsFinish() {
+	return new Promise(async function (resolve, reject) {
+		var downloadCountChecker = setInterval(function () {
+			if (ongoingDownloads < 1 && downloadQueue.length < 1 && !isDownloading) {
+				resolve(clearInterval(downloadCountChecker));
+			}
+		}, 256);
+	});
+}
+
+module.exports = {
+	"download_file": function (url, filePath) {
+		return new Promise(async function (resolve, reject) {
+			var directory = path.dirname(filePath);
+			if (!(await fs.existsSync(directory))) await fsPromises.mkdir(directory, {"recursive": true});
+
+			var writeStream = fs.createWriteStream(filePath);
+			writeStream.on("finish", resolve);
+
+			httpsWrapper.getStream(url).then(function (response) {
+				response.pipe(writeStream);
+			}).catch(reject);
+		});
+	},
+	"process_queue": function (progressCB) {
+		return new Promise(async function (resolve, reject) {
+			totalDownloads = downloadQueue.length;
+			if (downloadQueue.length > 0) isDownloading = true;
+
+			if (progressCB) {
+				progressCB(0.0, 0, downloadQueue.length, totalDownloads);
+			}
+
+			while (downloadQueue.length > 0) {
+				if (ongoingDownloads < maxConcurrentDownloads) {
+					setImmediate(function (downloadData) {
+						module.exports.download_file(downloadData.url, downloadData.path).then(function () {
+							ongoingDownloads--;
+							if (downloadQueue.length < 1) {
+								isDownloading = false;
+							}
+						}).catch(function (error) {
+							ongoingDownloads--;
+							downloadQueue.push(downloadData);
+						});
+					}, downloadQueue.shift());
+					ongoingDownloads++;
+					let progress = (totalDownloads - (downloadQueue.length + ongoingDownloads)) / totalDownloads;
+					if (progressCB) {
+						progressCB(progress, ongoingDownloads, downloadQueue.length, totalDownloads);
+					}
+				} else {
+					await utilities.sleep(256);
+				}
+			}
+
+			waitUntilDownloadsFinish().then(function () {
+				if (progressCB) {
+					progressCB(1.0, 0, 0, totalDownloads);
+				}
+				resolve();
+			}).catch(reject);
+		});
+	},
+	"download_minecraft": function (directory, meta, manifestSource, progressCB) {
+		return new Promise(async function (resolve, reject) {
+			let gameManifest = JSON.parse(await httpsWrapper.get(manifestSource["versions_manifest"]));
+
+			let versionInManifest = gameManifest.versions.find(ver => ver.id == meta["version"]);
+
+			if (!versionInManifest) {
+				return reject(new Error("Minecraft version \"" + meta["version"] + "\" could not be found in given source. "));
+			}
+
+			if (!fs.existsSync(directory)) await fsPromises.mkdir(directory, {"recursive": true});
+
+			let versionDirectory = path.join(directory, "versions", meta["version"]);
+
+			if (!fs.existsSync(versionDirectory)) await fsPromises.mkdir(versionDirectory, {"recursive": true});
+			let versionManifest = JSON.parse(await httpsWrapper.get(versionInManifest.url));
+			fs.writeFileSync(path.join(versionDirectory, meta["version"] + ".json"), JSON.stringify(versionManifest, null, "\t"));
+
+			let librariesDirectory = path.join(directory, "libraries");
+
+			for (let l = 0; l < versionManifest.libraries.length; l++) {
+				let library = versionManifest.libraries[l];
+
+				if ("classifiers" in library.downloads) continue; // Attempt at skipping platform-specific code, hoping the game still starts.
+
+				let libraryFilePath = path.join(librariesDirectory, ...(library.downloads.artifact.path.split("/")));
+
+				downloadQueue.push({
+					"url": library.downloads.artifact.url,
+					"path": libraryFilePath
+				});
+			}
+
+			let assetsDirectory = path.join(directory, "assets");
+
+			let assetIndexesDirectory = path.join(assetsDirectory, "indexes");
+			if (!fs.existsSync(assetIndexesDirectory)) await fsPromises.mkdir(assetIndexesDirectory, {"recursive": true});
+
+			let assetIndexPath = path.join(assetIndexesDirectory, versionManifest.assetIndex.id + ".json");
+			let assetIndex = JSON.parse(await httpsWrapper.get(versionManifest.assetIndex.url));
+			fs.writeFileSync(assetIndexPath, JSON.stringify(assetIndex, null, "\t"));
+
+			let assetObjectsDirectory = path.join(assetsDirectory, "objects");
+			for (const assetLocation in assetIndex.objects) {
+				if (assetIndex.objects.hasOwnProperty(assetLocation)) {
+					let asset = assetIndex.objects[assetLocation];
+
+					let assetPath = [
+						asset.hash.substring(0, 2), asset.hash
+					];
+
+					downloadQueue.push({
+						"url": manifestSource["asset_base"] + assetPath.join("/"),
+						"path": path.join(assetObjectsDirectory, ...assetPath)
+					});
+				}
+			}
+
+			downloadQueue.push({
+				"url": versionManifest.downloads.client.url,
+				"path": path.join(versionDirectory, meta["version"] + ".jar")
+			});
+
+			if (!isDownloading) await module.exports.process_queue(progressCB);
+
+			resolve(true);
+		});
+	}
+};

lib/util/functions.js

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+module.exports = {
+	"sleep": function (delay) {
+		return new Promise(function (resolve, reject) {
+			setTimeout(resolve, delay);
+		});
+	}
+};

lib/util/https.js

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+const https = require("https");
+
+module.exports = {
+	"get": function (url) {
+		return new Promise(function (resolve, reject) {
+			var request = https.get(url, function (response) {
+				if (response.statusCode != 200) return reject(new Error("Response status code wasn't 200. "));
+
+				response.setEncoding("utf-8");
+				let body = "";
+
+				response.on("data", function (chunk) {
+					body += chunk;
+				});
+
+				response.on("end", function () {
+					resolve(body);
+				});
+			});
+
+			request.on("error", reject);
+		});
+	},
+	"getStream": function (url) {
+		return new Promise(function (resolve, reject) {
+			var request = https.get(url, function (response) {
+				if (response.statusCode != 200) return reject(new Error("Response status code wasn't 200"));
+
+				resolve(response);
+			});
+
+			request.on("error", reject);
+		});
+	}
+};