With the old pre-boxpointer summary notifications, it sort of made
sense that the summary notification actor was a child of the message
tray. But there's no reason for that now, and in fact, it ends up
requiring special cases in some places since hovering over the summary
notification counts as hovering over the tray. So, fix this.
https://bugzilla.gnome.org/show_bug.cgi?id=655813
446910cb10
