Start using MessageTray.Source directly instead of having to subclass it

For most subclasses, this is a direct swap -- a lot of the time, the
constructor was a blank class that override createNotificationIcon,
and called _setSummaryIcon in _init.

https://bugzilla.gnome.org/show_bug.cgi?id=661236

js/ui/shellMountOperation.js

@@ -243,6 +243,7 @@ const ShellMountPasswordSource = new Lang.Class({
         this.parent(strings[0]);
 
         this._notification = new ShellMountPasswordNotification(this, strings, icon, reaskPassword);
+        this._setSummaryIcon(icon);
 
         // add ourselves as a source, and popup the notification
         Main.messageTray.add(this);