weston: add touchscreen_calibrator option

Add an option to enable the touchscreen calibrator interface. This is a
global on/off toggle, in lack of more fine-grained access restrictions.

As Weston should not hardcode system specifics, the actual permanent
saving of a new calibration is left for a user supplied script or a
program. Usually this script would write an appropriate udev rule to set
LIBINPUT_CALIBRATION_MATRIX for the touch device.

Co-developed by Louis-Francis and Pekka.

v2:
- use syspath instead of devpath

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v1 Tested-by: Matt Hoosier <matt.hoosier@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>