backends: extend tablet device checks

The Clutter X11 backend can't drop CLUTTER_PEN_DEVICE and
CLUTTER_ERASER_DEVICE in favor of CLUTTER_TABLET_DEVICE without
losing information (as the driver will create one device for each).
So make MetaInputSettings cater for both sets of device types.

https://bugzilla.gnome.org/show_bug.cgi?id=773779