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.cpp

@@ -400,39 +400,15 @@ void PluginScriptLanguage::reload_tool_script(const Ref<Script> &p_script, bool
 }
 
 void PluginScriptLanguage::lock() {
-#ifndef NO_THREADS
-	if (_lock) {
-		_lock->lock();
-	}
-#endif
+	_lock.lock();
 }
 
 void PluginScriptLanguage::unlock() {
-#ifndef NO_THREADS
-	if (_lock) {
-		_lock->unlock();
-	}
-#endif
+	_lock.unlock();
 }
 
 PluginScriptLanguage::PluginScriptLanguage(const godot_pluginscript_language_desc *desc) :
 		_desc(*desc) {
 	_resource_loader = Ref<ResourceFormatLoaderPluginScript>(memnew(ResourceFormatLoaderPluginScript(this)));
 	_resource_saver = Ref<ResourceFormatSaverPluginScript>(memnew(ResourceFormatSaverPluginScript(this)));
-
-// TODO: totally remove _lock attribute if NO_THREADS is set
-#ifdef NO_THREADS
-	_lock = NULL;
-#else
-	_lock = Mutex::create();
-#endif
-}
-
-PluginScriptLanguage::~PluginScriptLanguage() {
-#ifndef NO_THREADS
-	if (_lock) {
-		memdelete(_lock);
-		_lock = NULL;
-	}
-#endif
 }