[HTML5] Libraries refactor for linting.

Initial work to make liniting easier.

This includes:
- Rename http_request.js to library_godot_http_request.js.
- Rename externs.js to engine.externs.js.
- New library_godot_runtime.js (GodotRuntime) wraps around emscripten
  functions.
- Refactor of XMLHttpRequest handler in engine/preloader.js.
- Few fixes to bugs spotted by early stage linting.

platform/javascript/js/engine/preloader.js

@@ -0,0 +1,129 @@
+var Preloader = /** @constructor */ function() { // eslint-disable-line no-unused-vars
+
+	const loadXHR = function(resolve, reject, file, tracker, attempts) {
+		const xhr = new XMLHttpRequest();
+		tracker[file] = {
+			total: 0,
+			loaded: 0,
+			final: false,
+		};
+		xhr.onerror = function() {
+			if (attempts <= 1) {
+				reject(new Error("Failed loading file '" + file + "'"));
+			} else {
+				setTimeout(function () {
+					loadXHR(resolve, reject, file, tracker, attempts - 1);
+				}, 1000);
+			}
+		};
+		xhr.onabort = function() {
+			tracker[file].final = true;
+			reject(new Error("Loading file '" + file + "' was aborted."));
+		};
+		xhr.onloadstart = function(ev) {
+			tracker[file].total = ev.total;
+			tracker[file].loaded = ev.loaded;
+		};
+		xhr.onprogress = function(ev) {
+			tracker[file].loaded = ev.loaded;
+			tracker[file].total = ev.total;
+		};
+		xhr.onload = function() {
+			if (xhr.status >= 400) {
+				if (xhr.status < 500 || attempts <= 1) {
+					reject(new Error("Failed loading file '" + file + "': " + xhr.statusText));
+					xhr.abort();
+				} else {
+					setTimeout(function () {
+						loadXHR(resolve, reject, file, tracker, attempts - 1);
+					}, 1000);
+				}
+			} else {
+				tracker[file].final = true;
+				resolve(xhr);
+			}
+		};
+		// Make request.
+		xhr.open('GET', file);
+		if (!file.endsWith('.js')) {
+			xhr.responseType = 'arraybuffer';
+		}
+		xhr.send();
+	};
+
+	const DOWNLOAD_ATTEMPTS_MAX = 4;
+	const loadingFiles = {};
+	const lastProgress = { loaded: 0, total: 0 };
+	let progressFunc = null;
+
+	const animateProgress = function() {
+
+		var loaded = 0;
+		var total = 0;
+		var totalIsValid = true;
+		var progressIsFinal = true;
+
+		Object.keys(loadingFiles).forEach(function(file) {
+			const stat = loadingFiles[file];
+			if (!stat.final) {
+				progressIsFinal = false;
+			}
+			if (!totalIsValid || stat.total === 0) {
+				totalIsValid = false;
+				total = 0;
+			} else {
+				total += stat.total;
+			}
+			loaded += stat.loaded;
+		});
+		if (loaded !== lastProgress.loaded || total !== lastProgress.total) {
+			lastProgress.loaded = loaded;
+			lastProgress.total = total;
+			if (typeof progressFunc === 'function')
+				progressFunc(loaded, total);
+		}
+		if (!progressIsFinal)
+			requestAnimationFrame(animateProgress);
+	}
+
+	this.animateProgress = animateProgress;
+
+	this.setProgressFunc = function(callback) {
+		progressFunc = callback;
+	}
+
+
+	this.loadPromise = function(file) {
+		return new Promise(function(resolve, reject) {
+			loadXHR(resolve, reject, file, loadingFiles, DOWNLOAD_ATTEMPTS_MAX);
+		});
+	}
+
+	this.preloadedFiles = [];
+	this.preload = function(pathOrBuffer, destPath) {
+		let buffer = null;
+		if (typeof pathOrBuffer === 'string') {
+			var me = this;
+			return this.loadPromise(pathOrBuffer).then(function(xhr) {
+				me.preloadedFiles.push({
+					path: destPath || pathOrBuffer,
+					buffer: xhr.response
+				});
+				return Promise.resolve();
+			});
+		} else if (pathOrBuffer instanceof ArrayBuffer) {
+			buffer = new Uint8Array(pathOrBuffer);
+		} else if (ArrayBuffer.isView(pathOrBuffer)) {
+			buffer = new Uint8Array(pathOrBuffer.buffer);
+		}
+		if (buffer) {
+			this.preloadedFiles.push({
+				path: destPath,
+				buffer: pathOrBuffer
+			});
+			return Promise.resolve();
+		} else {
+			return Promise.reject(new Error("Invalid object for preloading"));
+		}
+	};
+};