LibGUI: Add a Widget::did_layout() that gets called after layout

This will allow widgets to implement post-layout behaviors.

Libraries/LibGUI/Widget.cpp

@@ -245,6 +245,7 @@ void Widget::do_layout()
     if (!m_layout)
         return;
     m_layout->run(*this);
+    did_layout();
     update();
 }
 

Libraries/LibGUI/Widget.h

@@ -283,6 +283,7 @@ protected:
 
     virtual void custom_layout() {}
     virtual void did_change_font() {}
+    virtual void did_layout() {}
     virtual void paint_event(PaintEvent&);
     virtual void resize_event(ResizeEvent&);
     virtual void show_event(ShowEvent&);