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/visual_script/visual_script.h

@@ -507,7 +507,7 @@ public:
 
 	static VisualScriptLanguage *singleton;
 
-	Mutex *lock;
+	Mutex lock;
 
 	bool debug_break(const String &p_error, bool p_allow_continue = true);
 	bool debug_break_parse(const String &p_file, int p_node, const String &p_error);