Demo Recording/Playback implemented

[Ingo Ruhnke]

Hiho,

I have now implemented demo recording and playback, seems to work
mostly, but fast-forward in the playback causes it to get out of sync,
not sure why, most likly an update() function that uses real-time
instead of game-time or a draw() function that isn't constant
(declaring it const wouldn't help here without rewriting parts of the
sprite class and if mutable works on win32 is another question...).

Anyway, this means that we can now record successfull plays of levels
and use them for more or less automatic valiadation, at least for a
short time, since demos will get out of sync if action behaviour
changes. Makes also bug tracking of action bugs a lot easier.

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