messageTray: Add focusTrap

The summary container will trap the focus if any sources are present,
making the message tray menu unreachable by keynav. Apply the same
hack as in searchDisplay and add a focusTrap to move the focus manually
as necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=707799
Bump version to 3.13.92

Update NEWS.