LibGUI: Rename ModelClient::on_model_update() => model_did_update()

This follows the typical client callback naming scheme used elsewhere
and doesn't collide with the "on_foo" Function hook convention.

Libraries/LibGUI/SortingProxyModel.cpp

@@ -50,7 +50,7 @@ SortingProxyModel::~SortingProxyModel()
     m_source->unregister_client(*this);
 }
 
-void SortingProxyModel::on_model_update(unsigned flags)
+void SortingProxyModel::model_did_update(unsigned flags)
 {
     resort(flags);
 }