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
Introduce the Universal Access status indicator as designed, modeled
after the similar UI provided by g-s-d. This indicator allows the user
to change rapidly the keyboard and mouse behaviour (sticky keys, slow
keys, bounce keys, mouse keys), as well as the enabled ATs (magnifier,
screen reader, screen keyboard) and the HighContrast Gtk theme.
https://bugzilla.gnome.org/show_bug.cgi?id=624916
