Introduce the base class for compositor extensions
When the compositor initializes a client, it may need to lookup already installed extensions on the toplevel surface, for example to find out whether the xdg_decoration has been installed on the xdg_toplevel surface.
In most cases, we can't use the manager object for this purpose because the created objects may outlive it.
This change introduces the base class for all compositor extensions and wayland objects.
The new classes provide a generic mechanism for registration and looking up installed compositor extensions.
The new compositor extension class is heavily inspired after a similar class in QtWaylandCompositor.
This MR is still WIP because my xdg-shell patches haven't landed yet.