common: Add xdevice_is_libinput()

This function pokes a libinput-driver-specific property, and can be used to
find out whether a device uses the libinput driver or not.

This will be used to determine whether a device is to be managed by mutter,
or by g-s-d itself.

https://bugzilla.gnome.org/show_bug.cgi?id=742593