register commercial-skip and audio-sync-adjustment key bindings

[Robert Tsai <rtsai1111>]

TOGGLEAUDIOSYNC is not registered in the TV Playback key bindings; this patch registers a key binding for it. I chose "Y" because I *think* playing back recordings ought to be disjoint with live TV, but I might be wrong, in which case a different default key should be used. I'm a little confused here because some actions (e.g., SIGNALMON) verify the "in Live TV" state before doing anything, while others that seem non-Live-TV-ish don't similarly validate their state (QUEUETRANCODE, SPEEDINC, etc.).

This patch also adds a new CYCLECOMMSKIP action and registers a key binding for it. I chose 'R' because I think LiveTV and Recorded TV playback should be able to re-use keys without colliding with each other, but maybe the "real-time" commercial flagging enables comm-skipping during LiveTV, in which case something other than "R" should be chosen.

I also do a little bit of cleanup; the "commercialskipmethod" variable appears to be unused ...

[Robert Tsai <rtsai1111>]

also update keys.txt

[cpinkham]

(In [9188]) Add keybindings for toggling the audio sync adjustment controls and for cycling through the commercial skip modes (auto/on, off, notify). Closes #1271 by applying patch by Robert Tsai.