Make the device <-> seat association permanent, and move it into
MetaWaylandInputDevice. A device will never be disassociated with a
seat, so there is no point in unsetting it.
https://bugzilla.gnome.org/show_bug.cgi?id=771305
Add a new object class, MetaWaylandInputDevice, and make all device
classes (pointer, keyboard, touch) inherit it. In the future common
functionality may be placed there.
https://bugzilla.gnome.org/show_bug.cgi?id=771305