VisualBuilder: Share code for setting the selected widget.

This fixes an issue where the properties window didn't update when selecting
a new widget using the keyboard shortcuts. :^)

Applications/VisualBuilder/VBForm.h

@@ -35,6 +35,7 @@ protected:
 
 private:
     void grabber_mousedown_event(GMouseEvent&, VBWidget&, Direction grabber);
+    void set_selected_widget(VBWidget*);
 
     String m_name;
     int m_grid_size { 5 };