volume: Ignore slider changes we initiated ourselves

volume: Ignore slider changes we initiated ourselves

Commit 21e14bd46f9774e6c0146cb2169d938e782dba3c fixed this for the
brightness slider, but we have the same problem for volume too. When the
volume is muted - for example in Settings or via a media key, we update
the slider to '0' to indicate this visually. But we also actually invoke
the slider's callback to *set* the volume to zero. That means that the
previous level is overwritten so it can't be restored when unmuting.

The fix is the same - when we update the slider internally ourselves,
don't call the signal handler.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1557