add new vertex slide transform operator, different from the existing vertex slide tool based on user feedback.
- no 2-step select edge, then slide.  Instead you can slide and select the edge at the same time.
- ability to slide multiple verts at one.

supports proportional option for vertex slide and flipping, both matching edge slide functionality.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53767 954f8c5b-7b00-dc11-b283-0030488c597c