LibGUI: Add ModelClient abstract class and allow registering clients

This solves a problem where the SortingProxyModel doesn't
receive the on_update call because other code overwrote
the handler later on.

Libraries/LibGUI/Model.cpp

@@ -55,8 +55,9 @@ void Model::for_each_view(Function<void(AbstractView&)> callback)
 
 void Model::did_update(unsigned flags)
 {
-    if (on_update)
-        on_update();
+    for (auto* client : m_clients)
+        client->on_model_update(flags);
+
     for_each_view([&](auto& view) {
         view.did_update_model(flags);
     });
@@ -82,4 +83,14 @@ bool Model::accepts_drag(const ModelIndex&, const StringView&)
     return false;
 }
 
+void Model::register_client(ModelClient& client)
+{
+    m_clients.set(&client);
+}
+
+void Model::unregister_client(ModelClient& client)
+{
+    m_clients.remove(&client);
+}
+
 }