changes to mirror tools

- give feedback on how many mirror verts succeed/fail (for select mirror, shape key mirror, weight mirror)
... when a mirror failed it was confusing and not obvious what was going on.

- slight change to select mirror, now center vertices will remain selected.

- speedup to EDBM_verts_mirror_cache_begin, cache customdata layer offset.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57245 954f8c5b-7b00-dc11-b283-0030488c597c