changes to ruler
- click-drag adds a ruler if there are none.
- pressing enter stores the ruler for re-use when activating again (saves as a grease-pencil layer).
- add to toolbar.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55120 954f8c5b-7b00-dc11-b283-0030488c597c