LibGUI: Make GTreeView fire the on_selection hook

GTreeView was forgetting to call to base in did_update_selection().
This prevented GAbstractView from firing the on_selection hook and we
ended up with only the on_selection_change hook firing.

Libraries/LibGUI/GTreeView.cpp

@@ -248,6 +248,7 @@ void GTreeView::did_update_model()
 
 void GTreeView::did_update_selection()
 {
+    GAbstractView::did_update_selection();
     ASSERT(model());
     auto index = selection().first();
     if (!index.is_valid())