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.

source/blender/editors/mesh/mesh_ops.c

@@ -172,6 +172,7 @@ void ED_operatortypes_mesh(void)
 #endif
 
 	WM_operatortype_append(MESH_OT_symmetrize);
+	WM_operatortype_append(MESH_OT_symmetry_snap);
 
 #ifdef WITH_GAMEENGINE
 	WM_operatortype_append(MESH_OT_navmesh_make);