calendar: Add NotificationSection to message list

Display notifications that have not been dismissed in the message
list - eventually this will replace the existing message tray summary.
Notification messages show icon, title and one line of the body and
can be clicked to activate the default action. However they cannot be
expanded, so other actions or the full body text are not accessible
in this mode.

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

js/ui/main.js

@@ -159,8 +159,8 @@ function _initializeUI() {
     if (LoginManager.canLock())
         screenShield = new ScreenShield.ScreenShield();
 
-    panel = new Panel.Panel();
     messageTray = new MessageTray.MessageTray();
+    panel = new Panel.Panel();
     keyboard = new Keyboard.Keyboard();
     notificationDaemon = new NotificationDaemon.NotificationDaemon();
     windowAttentionHandler = new WindowAttentionHandler.WindowAttentionHandler();