clean up ArdourWindow

- gtk callbacks are protected, not public
- virtual d'tor (destroy window by ArdourWindow ptr)

gtk2_ardour/ardour_window.h

@@ -39,18 +39,19 @@ namespace WM {
  */
 class ArdourWindow : public Gtk::Window, public ARDOUR::SessionHandlePtr, public Gtkmm2ext::VisibilityTracker
 {
-  public:
+public:
 	ArdourWindow (std::string title);
 	ArdourWindow (Gtk::Window& parent, std::string title);
-	~ArdourWindow();
+	virtual ~ArdourWindow();
 
+protected:
 	bool on_focus_in_event (GdkEventFocus*);
 	bool on_focus_out_event (GdkEventFocus*);
 	bool on_delete_event (GdkEventAny *);
 	bool on_key_press_event (GdkEventKey*);
 	void on_unmap ();
 
-  private:
+private:
 	WM::ProxyTemporary* proxy;
 	void init ();
 };