Snap to Symmetry (editmesh tool)

Update from a script I had in 2.4x, useful if you have mesh data which is _almost_ symmetrical,
there was no good way to automatically make the minor adjustments to make the mesh fully symmetrical.

Options for...
- symmetry-axis.
- blending between the +/- side.
- center mid verts.

Access from Mesh menu.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57590 954f8c5b-7b00-dc11-b283-0030488c597c