Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

editor/audio_stream_preview.cpp

@@ -197,7 +197,8 @@ Ref<AudioStreamPreview> AudioStreamPreviewGenerator::generate_preview(const Ref<
 	preview->preview->length = len_s;
 
 	if (preview->playback.is_valid()) {
-		preview->thread = Thread::create(_preview_thread, preview);
+		preview->thread = memnew(Thread);
+		preview->thread->start(_preview_thread, preview);
 	}
 
 	return preview->preview;
@@ -218,7 +219,8 @@ void AudioStreamPreviewGenerator::_notification(int p_what) {
 		for (Map<ObjectID, Preview>::Element *E = previews.front(); E; E = E->next()) {
 			if (!E->get().generating) {
 				if (E->get().thread) {
-					Thread::wait_to_finish(E->get().thread);
+					E->get().thread->wait_to_finish();
+					memdelete(E->get().thread);
 					E->get().thread = nullptr;
 				}
 				if (!ObjectDB::get_instance(E->key())) { //no longer in use, get rid of preview