Make it possible to sort a GTableModel by column+order.

This is accomplished by putting a GSortingProxyTableModel between the model
and the view. It's pretty simplistic but it works for this use case. :^)

LibGUI/GTableModel.cpp

@@ -27,6 +27,8 @@ void GTableModel::for_each_view(Function<void(GTableView&)> callback)
 
 void GTableModel::did_update()
 {
+    if (on_model_update)
+        on_model_update(*this);
     for_each_view([] (GTableView& view) {
         view.did_update_model();
     });