A new command, Filer_Visibility, allows various special OS applications such as !Boot to be hidden from the Filer display, modelled on
the "Hide OS files" option in Windows Explorer.
Hides !Boot, !Choices, !Scrap, !Territory (the movability of !Scrap and !Choices requires DesktopBoot-1_31 or later)
This is only a GUI display trick, the underlying APIs all still think the directories are there.
When launching a Filer_Action the current setting is passed in the flags.
Born out of a discussion about protecting !Boot and went through a few iterations following user feedback
