Completely re-engineered version of Loopy, courtesy of Lambros

Completely re-engineered version of Loopy, courtesy of Lambros
Lambrou. Now capable of handling triangular and hexagonal grids as
well as square ones, and then a number of semiregular plane tilings
and duals of semiregular ones. In fact, most of the solver code
supports an _arbitrary_ planar graph (well, provided both the graph
and its dual have no self-edges), so it could easily be extended
further with only a little more effort.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@8162 cda61777-01e9-0310-a592-d414129be87e