messageTray: fix clicking on links again

The change to make Notification an StButton (06d2c0af, bug 642978)
broke links, because the link actor would ignore the
button-press-event, allowing the notification actor to receive it and
get a pointer grab, and so the link actor would never see the
button-release-event. Fix that by accepting and discarding the
button-press-event.

https://bugzilla.gnome.org/show_bug.cgi?id=645613