Allow setting which sounds are muted by silent mode.

All sound categories are supported, but not all are actually shown in
the menu; I've only shown the ones where it seems meaningful to silence
them.

I was undecided where to put this in the sound menu.  It most naturally
goes under "Silent mode", but I pushed it near the bottom of the section
since the other sound settings are much more frequently used.

There are significantly different uses of "silent mode":

- I'm asleep.  I don't want notifications or calls ringing and waking me
  up, but I do want alarms to wake me up.  (Android almost does this by
  default--it still plays music in silent mode, which is annoying with
  the "android randomly plays classical music" bug.)
- I'm in a place where I don't want my device making noise; for example,
  watching a movie.  Nothing should play, including music and alarms.

This doesn't make it easy to switch between uses, but it's a start.  Note
that "silent mode silences alarms" is also available in the clock app, buried
very deep in options; these use the same setting and will stay in sync.