wacom: Fix device filtering in calibration UI

We receive ClutterEvents, not GdkEvents for button presses on
ClutterActors, so use the correct functions to filter devices.

This also fixes the offset used to access the coordinates of the
events. We were actually using the pointer to the source and x
struct members instead of x and y.

https://bugzilla.gnome.org/show_bug.cgi?id=707784