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/pluginscript/pluginscript_language.h

@@ -53,7 +53,7 @@ class PluginScriptLanguage : public ScriptLanguage {
 	const godot_pluginscript_language_desc _desc;
 	godot_pluginscript_language_data *_data;
 
-	Mutex *_lock;
+	Mutex _lock;
 	SelfList<PluginScript>::List _script_list;
 
 public:
@@ -126,7 +126,7 @@ public:
 	void unlock();
 
 	PluginScriptLanguage(const godot_pluginscript_language_desc *desc);
-	virtual ~PluginScriptLanguage();
+	virtual ~PluginScriptLanguage() {}
 };
 
 #endif // PLUGINSCRIPT_LANGUAGE_H