fix for issues with ruler
- disabling ruler snap wasn't setting the points depth back to its previous value before snapping. 
- adding a new ruler uses the depth of the previous active ruler (fallback to view center as before)
- deleting the ruler now sets the prev/next ruler active.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56241 954f8c5b-7b00-dc11-b283-0030488c597c