[quake3] feature: advanced demo navigation

[monk at rq3.com]

> And it's always nice to be able to
> pop around in demos in other games, too.

LIKE in other games.  Not have ioq3 play demos from other games.  But
comparing to, say, the playback features of a racing car game, for
example.  They tend to have the features I described.

Sorry, I re-read that and noticed that line made me look insane.  No more
spamses I promises.

Monk.