Diff view and controls for history/snapshot listing

[onecrayon]

Icons to side allow you to choose to/from diff targets. If there are multiple decks in the list between the two, they get collapsed and a third icon is added to clear the selection (and indicating the number of hidden snapshots). From has red colored cards and dice for things that were removed. To has green colored cards and dice for things that were added.

Would also like to have a way to "revert" to a snapshot (apply the snapshot's cards/PB/dice over top the current latest save; maybe save a snapshot just prior?).