Preview-modes: Add 'Disable point-and-click'

As Janek pointed out (http://lists.gnu.org/archive/html/lilypond-user/2013-10/msg00936.html) it is desirable to be able to disable point-and-click also in preview mode.

I know this changes the way Preview Mode worked, but I think it's a valid request.
However I returned the usual mode: Default (unchecked) means: Point-and-Click is *on*.