GTreeView: Always repaint in response to did_update_model()

This could definitely be more efficient, but this is more correct than
doing nothing at all. :^)

Libraries/LibGUI/GTreeView.cpp

@@ -244,6 +244,7 @@ void GTreeView::did_update_model()
 {
     GAbstractView::did_update_model();
     update_content_size();
+    update();
 }
 
 void GTreeView::did_update_selection()