Add volume indicator

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

src/shell-global.h

@@ -130,6 +130,9 @@ void shell_global_run_at_leisure (ShellGlobal         *global,
                                   gpointer             user_data,
                                   GDestroyNotify       notify);
 
+void shell_global_play_theme_sound (ShellGlobal       *global,
+                                    const char        *name);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */