This adds a new preference to Playback prefs, VariSpeedPlay, on by default, which makes it possible to vary the playback speed slider whilst audio is playing using play at speed. The code uses the Scrubbing playback engine.
This code is not final, but is already useful. Known issues:
1- Playback does not stop and pop-up the Play at Speed button at the end of playing the selection.
2- The scrub widget shows uselessly on the scrub ruler when in Play at Speed mode and the status bar proclaims 'Scrubbing'.
3- At low speeds the playback is clicky, with VariSpeedPlay, even if no changes to speed are being made.
If we can't fix both 1 and 2 by 2.3.0 release time, VariSpeedPlay will be made false by default.
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