LibGUI: Move widget registration to LibCore

This also moves Widget::load_from_json into Core::Object as a virtual
function in order to allow loading non-widget objects in GML (e.g.
BoxLayout).

Co-authored-by: Gunnar Beutner <gbeutner@serenityos.org>

Userland/Libraries/LibCore/Forward.h

@@ -27,6 +27,7 @@ class NetworkJob;
 class NetworkResponse;
 class Notifier;
 class Object;
+class ObjectClassRegistration;
 class ProcessStatisticsReader;
 class Socket;
 class SocketAddress;