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.
With everything now using GUI::FileIconProvider and therefore loading
icons embedded in the executable files, this information is now longer
being used.
We might have to think about this again if we want to allow .af files
with custom commands (e.g. shell scripts). Maybe those could get away
with just an "Icon" entry under "[App]", but currently there's only
"Executable" anyway.
