Move xdg_shell related functionality to a new meta-wayland-xdg-shell.c
and wl_shell related functionality to a new meta-wayland-wl-shell.c,
and adapt role object tree.

Common functionality related to the surface being drawn as a
MetaSurfaceActor was moved to a MetaWaylandSurfaceRoleActorSurface role.

The subsurface role GObject is made to inherit the actor surface GObject.

Shell surface hooks (configure, ping, close, popup done) were added to
a MetaWaylandSurfaceRoleShellSurface GObject which inherits the
surface actor role GObject.

The shell surface roles (xdg_surface, xdg_popup, wl_shell_surface) are
made to inherit the shell surface GObject and implement the relevant
API.

https://bugzilla.gnome.org/show_bug.cgi?id=757623

https://bugzilla.gnome.org/show_bug.cgi?id=763431