When a device is removed from the seat the events that this device may have
emitted just before being removed might still be in the stage events queue,
this may lead a to a crash because:

Once the device is removed, we dispose it and the staling event is
kept in queue and sent for processing at next loop.
During event processing we ask the backend to update the last device
with the disposed device
The device is disposed once the events referencing it, are free'd
The actual last device emission happens in an idle, but at this point
the device may have been free'd, and in any case will be still disposed
and so not providing useful informations.

To avoid this, once a device has been added/removed from the seat, we queue
ClutterDeviceEvent events to inform the stack that the device state has
changed, preserving the order with the other actual generated device events.
In this way it can't happen that we emit another event before that the
device has been added or after that it has been removed.

Fixes: #1345

(cherry picked from commit 9db289b4)