[HTML5] Refactor JS, threads support, closures.

- Refactored the Engine code, splitted across files.
- Use MODULARIZE option to build emscripten code into it's own closure.
- Enable lto support (saves ~2MiB in release).
- Enable optional closure compiler pass for JS and generated code.
- Enable optional pthreads support.
- Can now build with tools=yes (not much to see yet).
- Dropped some deprecated code for older toolchains.

platform/javascript/engine/loader.js

@@ -0,0 +1,33 @@
+var Loader = /** @constructor */ function() {
+
+	this.env = null;
+
+	this.init = function(loadPromise, basePath, config) {
+		var me = this;
+		return new Promise(function(resolve, reject) {
+			var cfg = config || {};
+			cfg['locateFile'] = Utils.createLocateRewrite(basePath);
+			cfg['instantiateWasm'] = Utils.createInstantiatePromise(loadPromise);
+			loadPromise = null;
+			Godot(cfg).then(function(module) {
+				me.env = module;
+				resolve();
+			});
+		});
+	}
+
+	this.start = function(preloadedFiles, args) {
+		var me = this;
+		return new Promise(function(resolve, reject) {
+			if (!me.env) {
+				reject(new Error('The engine must be initialized before it can be started'));
+			}
+			preloadedFiles.forEach(function(file) {
+				Utils.copyToFS(me.env['FS'], file.path, file.buffer);
+			});
+			preloadedFiles.length = 0; // Clear memory
+			me.env['callMain'](args);
+			resolve();
+		});
+	}
+};