GModel: Rename on_model_update(GModel&) => on_update()

Just simplifying the API of this hook a little bit.

Applications/FileManager/DirectoryView.cpp

@@ -58,7 +58,7 @@ DirectoryView::DirectoryView(GWidget* parent)
 
     m_item_view->set_model_column(GDirectoryModel::Column::Name);
 
-    m_table_view->model()->on_model_update = [this](auto&) {
+    m_table_view->model()->on_update = [this] {
         set_status_message(String::format("%d item%s (%u byte%s)",
             model().row_count(),
             model().row_count() != 1 ? "s" : "",

Libraries/LibGUI/GModel.cpp

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

Libraries/LibGUI/GModel.h

@@ -67,7 +67,7 @@ public:
     void register_view(Badge<GAbstractView>, GAbstractView&);
     void unregister_view(Badge<GAbstractView>, GAbstractView&);
 
-    Function<void(GModel&)> on_model_update;
+    Function<void()> on_update;
     Function<void(const GModelIndex&)> on_selection_changed;
 
 protected:

Libraries/LibGUI/GSortingProxyModel.cpp

@@ -7,7 +7,7 @@ GSortingProxyModel::GSortingProxyModel(NonnullRefPtr<GModel>&& target)
     : m_target(move(target))
     , m_key_column(-1)
 {
-    m_target->on_model_update = [this](GModel&) {
+    m_target->on_update = [this] {
         resort();
     };
 }