calibrator: Make mouse button optional

calibrator: Make mouse button optional

When calibrating touchscreen with weston-calibrator, you can use the mouse to
click on the cross which is recorded as a touch event. This event is used to
compute the final calibration of the touchscreen which results in invalid
touchscreen calibration and broken touchscreen behaviour.

In order to avoid to use the mouse in weston-calibrator, we disable mouse
operation by default and add a parameter "--enable-mouse" to enable it.

Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>