[ui] Make ui_choose_file threadsafe

[EspeuteClement]

This allows using the hl.UI.loadFile/saveFile from another thread, which allow async dialog from haxe code. Async dialog code is needed for hide_hl as it fixes performance on GSync monitors while a dialog is open, because it allows the main app loop to not block. The api of the UI functions has not changed.

example code to use the dialog in an async way, that code can be implemented in the projects :

	public function chooseFileAsync(opts: hl.UI.FileOptions, onSelect: Null<String> -> Void, save = false) {
		var events = sys.thread.Thread.current().events;
		sys.thread.Thread.create(() -> {
			var path = save ? hl.UI.saveFile(opts) : hl.UI.loadFile(opts);
			events.run(() -> onSelect(path));
		});
	}