We currently paint the 3 previewed cards using ImageWidget. This works
fine, but in order to preview a card hovering over a valid target card,
it will be easier to use the already-existing CardGame paint logic. So
this patch changes GamesSettings to display the preview as-is using a
CardGame.
This currently has exactly one setting: The background colour for card
games. My thinking is, it's better to not have a Settings application
for each individual game we include in the system, since most will only
have a small number of settings, all Settings windows have tabs anyway,
and I don't want to flood the Settings app list unnecessarily.
As for having a single setting for all the card games: it's nice when
things match. :^)
