Modernize Mutex

- Based on C++11's `mutex`
- 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`
- `BinaryMutex` added for special cases as the non-recursive version
- `MutexLock` now takes a reference. At this point the cases of null `Mutex`es are rare. If you ever need that, just don't use `MutexLock`.
- `ScopedMutexLock` is dropped and replaced by `MutexLock`, because they were pretty much the same.

modules/gdnative/nativescript/nativescript.h

@@ -121,9 +121,7 @@ class NativeScript : public Script {
 	String script_class_name;
 	String script_class_icon_path;
 
-#ifndef NO_THREADS
-	Mutex *owners_lock;
-#endif
+	Mutex owners_lock;
 	Set<Object *> instance_owners;
 
 protected:
@@ -238,7 +236,7 @@ private:
 	void _unload_stuff(bool p_reload = false);
 
 #ifndef NO_THREADS
-	Mutex *mutex;
+	Mutex mutex;
 
 	Set<Ref<GDNativeLibrary> > libs_to_init;
 	Set<NativeScript *> scripts_to_register;