Remove Smart Link action code, add a new action for Link Opposite and document a...

Remove Smart Link action code, add a new action for Link Opposite and document a bit the code.
Now things works like this:
Link: All tangents of same colour, straight link
Link: Some tangents of different colour, smart link (scale by -1 when needed)
Link Opposite: two tangents of the same colour, scale by -1 before link
Link Opposite: two tangents of different colour, straight link
Link Opposite: more than two tangents, action is not candidate.