Bugfix [#34645] NlaStrip Tweak Mode - No automatic action length sync on exit

Originally I was thinking of not restoring this functionality (see report for
further details). However, upon rechecking the code, it seems that there was
actually a define set up for this already, but which wasn't actually hooked up
yet. So, this commit basically exposes this option ("Sync Length") which ensures
that when exiting tweak mode on a NLA Strip, that strip instance (and not other
users of the same action) will be updated to include the newly added keyframes.
This option is not enabled by default, and shouldn't really be needed in most
("intended") workflows.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56314 954f8c5b-7b00-dc11-b283-0030488c597c