[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.
- Optional closure compiler run for JS and generated code.
- Enable lto support (saves ~2MiB in release).
- Can now build with tools=yes (not much to see yet).
- Dropped some deprecated code for older toolchains.
- Add onExit, and onExecute JS function.
- Add files drag and drop support.
- Add support for low precessor usage mode (via offscreen render, swap).

platform/javascript/audio_driver_javascript.h

@@ -37,6 +37,7 @@ class AudioDriverJavaScript : public AudioDriver {
 
 	float *internal_buffer;
 
+	int _driver_id;
 	int buffer_length;
 
 public:
@@ -56,6 +57,7 @@ public:
 	virtual void lock();
 	virtual void unlock();
 	virtual void finish();
+	void finish_async();
 
 	virtual Error capture_start();
 	virtual Error capture_stop();