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

[NicholasBishop at gmail.com]

Reviewers: bf-codereview_blender.org,

Description:
The symmetrize operation makes the input mesh elements symmetrical, but
unlike mirroring it only copies in one direction. The edges and faces
that cross the plane of symmetry are split as needed to enforce
symmetry.

The symmetrize operator can be controlled with the "direction" property,
which combines the choices of symmetry plane and
positive-negative/negative-positive. The enum for this is
BMO_SymmDirection.

Documentation:
http://wiki.blender.org/index.php/User:Nicholasbishop/Symmetrize


Please review this at https://codereview.appspot.com/6618059/

Affected files:
   source/blender/bmesh/CMakeLists.txt
   source/blender/bmesh/intern/bmesh_opdefines.c
   source/blender/bmesh/intern/bmesh_operator_api.h
   source/blender/bmesh/intern/bmesh_operators_private.h
   source/blender/bmesh/operators/bmo_symmetrize.c
   source/blender/editors/mesh/editmesh_tools.c
   source/blender/editors/mesh/mesh_intern.h
   source/blender/editors/mesh/mesh_ops.c