messageTray: Add 'source-removed' signal

While it is possible to keep track of removed sources by tracking
their summary items' actor:.destroy signal, a dedicated signal
mirroring the existing 'source-added' one is more convenient.

https://bugzilla.gnome.org/show_bug.cgi?id=693220

js/ui/messageTray.js

@@ -1817,6 +1817,8 @@ const MessageTray = new Lang.Class({
 
         summaryItem.destroy();
 
+        this.emit('source-removed', source);
+
         this._updateNoMessagesLabel();
 
         if (needUpdate)