sync: work-around players with broken edit list support

sync: work-around players with broken edit list support

This adds a preset key AlignAVStart that enables this work-around. When
enabled, blank frames are inserted or frames are dropped to force
alignment of the initial timestamp of every audio and video stream.
Aligning the start times minimizes the impact of broken edit list
support in players.

Closes #763.

Squashed:

sync: improve alignment when passthru audio is present

presets: enable AlignAVStart for General and Gmail presets

LinGui: Improve AlignAVStart tooltip

sync: avoid inserting a black frame < nominal frame duration

sync: fix start alignment when doing p-to-p encoding

sync: add comments