automount: add an AutomountManager class

The AutomountManager class is the low-level counterpart of the
previously introduced AutorunManager, and takes care of extracting the
list of valid mounts from a GVolume or a GDrive and mounting them,
provided a number of conditions and requirements are met.

AutomountManager also keeps track of the current session availability
(using the ConsoleKit and gnome-screensaver DBus interfaces) and
inhibits mounting if the current session is locked, or another session
is in use instead.

https://bugzilla.gnome.org/show_bug.cgi?id=653520

js/ui/autorunManager.js

@@ -118,6 +118,11 @@ AutorunManager.prototype = {
     },
 
     _onMountAdded: function(monitor, mount) {
+        // don't do anything if our session is not the currently
+        // active one
+        if (!Main.automountManager.ckListener.sessionActive)
+            return;
+
         let discoverer = new ContentTypeDiscoverer(Lang.bind (this,
             function (mount, contentTypes) {
                 this._transDispatcher.addMount(mount, contentTypes);