Show those TODO message also when using the toolbar or the popup menu

This should let the user know that something's not working instead of
offering him a not working dialog and winding him up this way. That
should make the behaviour of the program much easier to understand.

src/MainWindow.h

@@ -68,6 +68,7 @@ public slots:
     virtual void on_tree_selection_changed();
     virtual void on_add_field();
     virtual void on_edit_field();
+    virtual void on_delete_field();
 
     virtual void fileOpen( const QString & fileName );
     virtual void fileOpen();