Drop old semaphore implementation

- Removed platform-specific implementations.
- Now all semaphores are in-object, unless they need to be conditionally created.
- Similarly to `Mutex`, provided a dummy implementation for when `NO_THREADS` is defined.
- Similarly to `Mutex`, methods are made `const` for easy use in such contexts.
- Language bindings updated: `wait()` and `post()` are now `void`.
- Language bindings updated: `try_wait()` added.

Bonus:
- Rewritten the `#ifdef` in `mutex.h` to meet the code style.

platform/server/os_server.cpp

@@ -68,10 +68,6 @@ void OS_Server::initialize_core() {
 	crash_handler.initialize();
 
 	OS_Unix::initialize_core();
-
-#ifdef __APPLE__
-	SemaphoreOSX::make_default();
-#endif
 }
 
 Error OS_Server::initialize(const VideoMode &p_desired, int p_video_driver, int p_audio_driver) {