Project Pampa request: option to lock frame selection to the range

This means when you've got "Lock Frame Selection" option (which is
in the timeline next to the preview range button) you're not able
to go to the frames which are out of current frame range with your
mouse.

TODO: Make it so current frame slider also respects this setting?
      Not so much important for tonight.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60733 954f8c5b-7b00-dc11-b283-0030488c597c