`Restart' is now an undo-able action: it appends a move to the end

`Restart' is now an undo-able action: it appends a move to the end
of the undo list rather than destroying it. Partly this is because
accidental restarts are a real pain, and partly because it allows
you to compare the initial to the current state by restart-then-undo
which is handy in some puzzles.

In order to do this, I've introduced an additional per-entry field
in the undo list in the midend, which tracks which states were
created by `unusual' operations (Solve and Restart). The midend
takes care of suppressing animation and completion flashes during
transitions between a `special' state and its predecessor, relieving
the game backends of having to do it individually.

(This probably means I could remove some complexity in the
flash_time() functions in most backends, but I haven't done that in
this checkin.)

[originally from svn r5791]