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)

platform/javascript/audio_driver_javascript.h

@@ -59,7 +59,7 @@ public:
 			STATE_MAX,
 		};
 		Mutex mutex;
-		Thread *thread = nullptr;
+		Thread thread;
 		bool quit = false;
 		int32_t state[STATE_MAX] = { 0 };