MainWindow: initial integration of new QuickLoadDialog

This is an initial integration of the new QuickLoadDialog into LMMS.
One can either open up the dialog via Ctrl+L or the "Edit" menu.

The dialog is for testing only and not useful at all yet.

include/main_window.h

@@ -137,6 +137,8 @@ public slots:
 	void undo( void );
 	void redo( void );
 
+	void loadResource();
+
 	void toggleRecordAutomation( bool );
 
 protected: