We currently hard-code excluding Settings apps from the system menu.
This adds an "ExcludeFromSystemMenu" option to the AppFile configuration
to selectively exclude these apps, which all Settings app now set.
This is to allow selectively excluding a few Demo apps in a future
commit.
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. :^)
