Modernize Semaphore

- Based on C++11's `mutex` and `condition_variable`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`

core/bind/core_bind.cpp

@@ -2689,12 +2689,14 @@ void _Marshalls::_bind_methods() {
 
 Error _Semaphore::wait() {
 
-	return semaphore->wait();
+	semaphore.wait();
+	return OK; // Can't fail anymore; keep compat
 }
 
 Error _Semaphore::post() {
 
-	return semaphore->post();
+	semaphore.post();
+	return OK; // Can't fail anymore; keep compat
 }
 
 void _Semaphore::_bind_methods() {
@@ -2703,16 +2705,6 @@ void _Semaphore::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("post"), &_Semaphore::post);
 }
 
-_Semaphore::_Semaphore() {
-
-	semaphore = Semaphore::create();
-}
-
-_Semaphore::~_Semaphore() {
-
-	memdelete(semaphore);
-}
-
 ///////////////
 
 void _Mutex::lock() {