messageTray: Make Notification._onClicked handler public

When we will start to show notifications in the date drop-down, we
will not use the actual notification actor, but construct our own UI
based on Calendar.Message. This is similar to what we already do in
the lock screen, except that in this case clicking the notification
should activate the default action.
So rename the existing _onClicked() method to activate() to make it
clear that such use is acceptable. While not strictly necessary, also
rename the corresponding signal to match.

https://bugzilla.gnome.org/show_bug.cgi?id=744817

js/ui/components/autorunManager.js

@@ -363,7 +363,7 @@ const AutorunNotification = new Lang.Class({
         return button;
     },
 
-    _onClicked: function() {
+    activate: function() {
         this.parent();
 
         let app = Gio.app_info_get_default_for_type('inode/directory', false);