Implement bits for startup notifications

This implements the bits needed for startup notifications via gtk_shell1 and partially xwayland (if the toplevel sets the id).

I intend to move the new events to a separate interface wanted to have the phoc code out so wlroots sees an implementation. Also once sorted out I'll look if we can move the compositor->shell forwarding bits into something like foreign_toplevel so it can live in wlroots (there's nothing phosh/phoc specific in there).