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.

core/command_queue_mt.cpp

@@ -108,11 +108,10 @@ CommandQueueMT::CommandQueueMT(bool p_sync) {
 
 	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
 
-		sync_sems[i].sem = SemaphoreOld::create();
 		sync_sems[i].in_use = false;
 	}
 	if (p_sync)
-		sync = SemaphoreOld::create();
+		sync = memnew(Semaphore);
 	else
 		sync = NULL;
 }
@@ -121,9 +120,5 @@ CommandQueueMT::~CommandQueueMT() {
 
 	if (sync)
 		memdelete(sync);
-	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
-
-		memdelete(sync_sems[i].sem);
-	}
 	memfree(command_mem);
 }