Galaxies: prevent creation of empty undo-chain items.

Galaxies: prevent creation of empty undo-chain items.

If you drag an arrow on to a square which is already filled in as part
of a completed region, or whose counterpart is filled in, or whose
counterpart is actually a dot, then the game can't actually place a
double arrow. Previously, it didn't find that out until execute_move
time, at which point it was too late to prevent a no-op action from
being placed on the undo chain.

Now we do those checks in interpret_move, before generating the move
string that tries to place the double arrow in the first place. So
execute_move can now enforce by assertion that arrow-placement moves
it gets are valid.