wayland: implement support for popup surfaces

Popup surfaces are mapped into override_redirect surfaces
of a DROPDOWN_MENU type, with the addition of a special pointer
grab.

https://bugzilla.gnome.org/show_bug.cgi?id=707863
wayland: heavily refactor pointer grabs

Grabs are now slice allocated structures that are handled by
whoever starts the grab. They contain a generic grab structure
with the interface and a backpointer to the MetaWaylandPointer.
The grab interface has been changed to pass full clutter events,
which allowed to remove the confusion between grab->focus and
pointer->focus. Invidual grabs are now required to keep their
focus, and choose whoever gets the events.

https://bugzilla.gnome.org/show_bug.cgi?id=707863
wayland: implement stacked popups

Allow multiple popups from the same clients to be stacked under
the same pointer grab.
This is necessary to implement submenus.

https://bugzilla.gnome.org/show_bug.cgi?id=707863
wayland: remove some wl_signal usage

It was a left-over from the initial code import from weston.

https://bugzilla.gnome.org/show_bug.cgi?id=707863