Cleanup: crossfade, muxer rewrite, fix add

[toots]

This PR cleans up some of the recent changes:

• Backport the fix for Pop/click at the end of fade out in 2.2.0 #3318 on the add operator
• Rewrite the muxer to be trivially simple!
• Rewrite the crossfade operator to take advantage of the Source.generate_from_multiple_sources logic.

One important change: fade.in and fade.out used to be triggered by track marks by default. However, with these changes and to be more in-tune with the new handling of track marks, the sources used to compute crossfade transitions do not have initial track marks unless they are in the underlying data.

Since most use of fade.in and fade.out are inside crossfade transitions it seems to make sense to change them to trigger immediately. The old behavior can still be obtained by passing track_sensitive=true.