Highlight most recently killed piece in boneyard

[arbrown]

Now that live pieces are technically visible in the boneyard, we should highlight the most recently killed one with the "last-killed" highlight instead of always highlighting the first.

In this picture, a second red pawn has just been taken, but the same pawn that was killed the turn prior retains its "last-killed" highlighting instead of the most recent one to turn dark/dead.

It's pedantic and unimportant, I know...