This is a fallout from some changes in MessageTray.Source, which now
requires either defining the iconName/iconType properties on it, or
implementing createNotificationIcon, and we're not doing any of those.
Fix it by storing the gicon of the source object and using a helper
method to create the icon actor on demand, to avoid any case when the
same actor might be added twice to different containers.
https://bugzilla.gnome.org/show_bug.cgi?id=678428
281b0a3e63