Main change is to rewrite the start/stop event handling to do fewer
passes and use simpler logic (it no longer has to track durations).
The temporary work audio codec is switched to flac; slightly lower hard
drive use and it avoids the 4-hour length limit of wav.
Some improvements to the video seeking when processing deskshare and
webcam videos, to avoid video dropouts.
Tweaks to the audio encoding quality settings.
Remove an unused function.
