libweston: notify_touch API to use weston_touch_device

Relay touch input events into libweston core through the
weston_touch_device, so that the core can tell which individual physical
device they come from.

This is necessary for supporting touchscreen calibration, where one
needs to process a single physical device at a time instead of the
aggregate of all touch devices on the weston_seat.

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>