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/guiutil.h

@@ -96,6 +96,11 @@ namespace GUIUtil
     bool GetStartOnSystemStartup();
     bool SetStartOnSystemStartup(bool fAutoStart);
 
+    /** Save window size and position */
+    void saveWindowGeometry(const QString& strSetting, QWidget *parent);
+    /** Restore window size and position */
+    void restoreWindowGeometry(const QString& strSetting, const QSize &defaultSizeIn, QWidget *parent);
+
     /** Help message for Bitcoin-Qt, shown with --help. */
     class HelpMessageBox : public QMessageBox
     {