Clutter device events are special events coming from the backend when an
input device is added or removed.

When such events are processed, we should make the seat to handle them by
calling vfunc that can be implemented by each backend and eventually
emitting the appropriate signal.

If a device is removed, we can also safely dispose it, as it can be
considered stale at this point.

!1371

(cherry picked from commit cf67c54f)