Add private gnome-volume-control library

The library is introspected, and should not require using
Pulseaudio directly.

With help from Giovanni Campagna <scampa.giovanni@gmail.com>
(introspection annotations, build fixes)

https://bugzilla.gnome.org/show_bug.cgi?id=629455

src/gnome-shell-plugin.c

@@ -296,6 +296,15 @@ add_statistics (GnomeShellPlugin *shell_plugin)
                                           NULL, NULL);
 }
 
+static void
+gvc_muted_debug_log_handler (const char     *log_domain,
+                             GLogLevelFlags  log_level,
+                             const char     *message,
+                             gpointer        data)
+{
+  /* Intentionally empty to discard message */
+}
+
 static void
 gnome_shell_plugin_start (MutterPlugin *plugin)
 {
@@ -355,6 +364,10 @@ gnome_shell_plugin_start (MutterPlugin *plugin)
   shell_plugin->gjs_context = gjs_context_new_with_search_path(search_path);
   g_strfreev(search_path);
 
+  /* Disable the gnome-volume-control debug */
+  g_log_set_handler ("Gvc", G_LOG_LEVEL_DEBUG,
+                     gvc_muted_debug_log_handler, NULL);
+
   /* Initialize the global object here. */
   global = shell_global_get ();