New puzzle: `Untangle', cloned (with the addition of random grid

New puzzle: `Untangle', cloned (with the addition of random grid
generation) from a simple but rather fun Flash game I saw this
morning.

Small infrastructure change for this puzzle: while most game
backends find the midend's assumption that Solve moves are never
animated to be a convenience absolving them of having to handle the
special case themselves, this one actually needs Solve to be
animated. Rather than break that convenience for the other puzzles,
I've introduced a flag bit (which I've shoved in mouse_priorities
for the moment, shamefully without changing its name).

[originally from svn r6097]