Rotate Quads (Ctrl+F > Rotate Quads)

Rotate the vertices of a face around. This way the user has full control over the triangulated face when using Fixed or Fixed Alternate triangulation methods.

@ideasman42 Hi. This is the missing bit for the triangulate related features. It works fine and it's nice to use it with the fixed method implemented in #5 . The only bit I'm not sure is if I should restrict this operator to quads only, or if it should work for ngons as well. The changes in the code are minimal, it's a matter of deciding what to implement