Created operator.

Created String::operator+=(const wchar_t), that was mark as Todo.

src/core/String.cpp

@@ -135,7 +135,7 @@ void String::operator+=(const String &s) {
 }
 
 void String::operator+=(const wchar_t c) {
-	// @Todo
+	_godot_string = godot::api->godot_string_operator_plus(&_godot_string, &(String(c)._godot_string));
 }
 
 bool String::operator<(const String &s) const {