Simplify the code, because the bit trickery was fairly nasty to follow.
KISS: flip one value at a time, either in or out.
Actually allow flipping *all* of the things via the UI, to help track
what the hell is actually happening when you touch a button.
Make some of 'em radio to make it clear when flipping one might affect
the other(s).
Brought on by https://www.mobileread.com/forums/showthread.php?t=358166
Notification: adds some functions so it can be used as
a notification manager.
Have various bits of code emitting events that may generate
notifications advertize themselves as the source for following
notifications.
Add a menu to allow selecting some subsets of sources
to show or hide.
