MessageTray: restore opacity when expanding a notification

MessageTray tweens both opacity and y to hide or show _notificationWidget,
but only y when expanding it. This means that an existing tween to hide
the notification will continue running, clearing the notification state.
If the hiding one completes before the showing one, the onComplete handler
will throw an exception (because the notification was nullified) and
therefore break the state tracking.

https://bugzilla.gnome.org/show_bug.cgi?id=683986