messageList: Use St.Bin as message container and use clutter to manage the list

When messages are added to the message list, we create a container for those,
however since now the messages are actor themselves we can just create a list
item actor that holds the message actor and refer to the message parent in order
to get their container.

This allows to remove the obj container map we used, using the native clutter
parent-child hierarchy and handle signal connections cleanly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559

js/ui/calendar.js

@@ -999,9 +999,10 @@ class NotificationSection extends MessageList.MessageListSection {
         if (!this.mapped)
             return;
 
-        for (let message of this._messages.keys())
+        this._messages.forEach(message => {
             if (message.notification.urgency != MessageTray.Urgency.CRITICAL)
                 message.notification.acknowledged = true;
+        });
     }
 
     _shouldShow() {