ruler/protractor for 3d viewport (apart of 3d printing features). work-in-progress.

- activate from spacebar search (3D Ruler) 
- ctrl-click adds new rulers
- clicking in the middle of a ruler, turns into protractor, dragging out of view snaps back to ruler.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55089 954f8c5b-7b00-dc11-b283-0030488c597c