messageTray: Ensure notifications are focused after they are expanded

If we focus notifications before they're expanded, the body and action
area won't be visible, and the can_focus members like the text entry
will not be able to be focused.

Ensure that all of the all actors that would be in an expanded notification
are visible before we attempt to focus them.

https://bugzilla.gnome.org/show_bug.cgi?id=698778