wacom: Fix event filtering in the calibrator

We were always getting the events from the core pointer instead
of the device itself, so threw all of them away.
We need to get the real source device from the event.

https://bugzilla.gnome.org/show_bug.cgi?id=707784
wacom: Update from gnome-settings-daemon
wacom: Reset the calibration before starting a new one

We shouldn't be using the old calibration values to create the
new ones, so reset the "area" settings before starting a new
calibration, and re-apply the saved calibration if the calibration
is cancelled or fails.

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