Opening and closing the device may result into XI2 grabs being cut short,
resulting into pad buttons being rendered ineffective, and other possible
misbehaviors. This is an XInput flaw that fell in the gap between XI1 and
XI2, and has no easy fix. It pays us for mixing both versions, I guess...

Work this around by keeping the XI1 XDevice attached to the
ClutterInputDevice, this way it will live long enough that this is not
a concern.

Investigation of this bug was mostly carried by Peter Hutterer, I'm just
the executing hand.

GNOME/mutter#7

Closes: #7