Notify panel update

Keep the notify panel visible while dead; if it overlaps the scoreboard area make it more transparent, if the game is over halve message duration.

Note: the scoreboard area has many empty spots (the gametype row on the top, the spectators list on the bottom) so the notify panel may become more transparent needlessly, unfortunately this misbehavior can't be properly handled any better. With the default alpha value of 0.5 messages are clearly readable anyway.

It fixes #2581 (closed)