messageList: Keep secondary actor when showing close button

messageList: Keep secondary actor when showing close button

Currently the secondary actor (if set) and the close button are
exclusive, that is the latter replaces the former on hover. As
the swapping feels rather busy and there's no real reason both
cannot be shown at the same time, keep the secondary actor always
visible.
A welcome side effect is that it no longer needs to be placed at
the end, so we can move the notification timestamp right next to
the corresponding title.

https://bugzilla.gnome.org/show_bug.cgi?id=775763