[Bf-codereview] Add BMesh and WM symmetrize operators (issue 6618059)

[brechtvanlommel at gmail.com]

There doesn't seem to be a perfect for any of the submenus (maybe
Mirror, but not really). I'd put it in the top level Mesh menu and the W
key specials menu.

https://codereview.appspot.com/6618059/