debugger: refactoring of the plugin interface

There's  some 'external' and some 'internal' part now. Other plugins
are only supposed the external interface.

src/plugins/debugger/moduleswindow.h

@@ -49,7 +49,6 @@ private slots:
     void setAlternatingRowColorsHelper(bool on) { setAlternatingRowColors(on); }
 
 private:
-    void resizeEvent(QResizeEvent *ev);
     void contextMenuEvent(QContextMenuEvent *ev);
     void setModel(QAbstractItemModel *model);