desktop-shell: disable opacity binding when modifier is none

This patch disables the opacity binding when the modifier is configured
to `none' in weston.ini, and thus supports use cases where one does not
want to have this binding.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>