_volumeChanged should not update the icon if the sink is muted.
Fixes the case when shell is started with a muted sink (_mutedChanged
is called before _volumeChanged).
https://bugzilla.gnome.org/show_bug.cgi?id=632868
Add volume control indicator which uses API from gnome-volume-control
to interact with PulseAudio and shows both input and output volumes.
Also adds a small wrapper around libcanberra in ShellGlobal, used by the
volume indicator to provide auditive feedback.
https://bugzilla.gnome.org/show_bug.cgi?id=629455