Bitcoin-Qt: save and restore position of debug window

- move the code for saving and restoring window positions from BitcoinGUI
  to GUIUtil, make it more generic and also use it for saving/restoring
  debug window positions

src/qt/bitcoingui.h

@@ -122,10 +122,6 @@ private:
     void createTrayIcon(bool fIsTestnet);
     /** Create system tray menu (or setup the dock menu) */
     void createTrayIconMenu();
-    /** Save window size and position */
-    void saveWindowGeometry();
-    /** Restore window size and position */
-    void restoreWindowGeometry();
 
 public slots:
     /** Set number of connections shown in the UI */