Untangle: add a 'snap to grid' user preference.

Requested by a user who otherwise found themself spending too much
time struggling to get lines nicely horizontal or vertical.

The implementation is easy, but the question is what size of grid is
appropriate. Untangle's own generated games are constructed by making
a planar graph drawn on an extremely coarse grid - but snapping to
_that_ grid would give away information about the puzzle solution, and
also, Untangle wouldn't know any similar information about graphs
generated by any other method.

So a better approach is to choose a size of grid that guarantees that
_any_ graph with n vertices can be drawn on it with nonintersecting
straight edges. That sounds like a tricky maths problem - but happily,
the solution is given in a book I already had a copy of. References in
a comment (plus a proof of a pedantic followup detail about multiple
planar embeddings).

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