wayland: Implement the server side bits of wl_touch_interface

Clutter touch events are translated into events being sent down
the interface resource, with the exception of FRAME/CANCEL events,
which are handled directly via an evdev event filter.

The seat now announces invariably the WL_SEAT_CAPABILITY_TOUCH
capability, this should be eventually updated as devices come and
go.

The creation of MetaWaylandTouchSurface structs is dynamic, attached
to the lifetime of first/last touch on the client surface, and only
if the surface requests the wl_touch interface. MetaWaylandTouchInfo
structs are created to track individual touches, and are locked to
a single MetaWaylandTouchSurface (the implicit grab surface) determined
on CLUTTER_TOUCH_BEGIN.

https://bugzilla.gnome.org/show_bug.cgi?id=724442