[airstrike] Airstrike sprite generators

[Eero Tamminen]

Hi,

> I've added sprite generators to Airstrike.  Biplane can also die again.

Ulf:
ATM only levels 0 and 3 use sprite generator for the biplane.


> Currently there's only "player" type of sprite generator

Others:
More will come now that the infrastructure is in place... :-)
There are several things that will profit from the use of sprite generator
objects i.e. controllling the sprite respawning from the level settings
instead of having it in code.  With Ulf's builtin Airstrike console, testing 
them will be fairly nice (I'm not sure whether I mentioned in the list the
readline support that I added to it? :)).

I'm not sure when I will have time to look into triggers (level end
condition, generator activation etc), but now that I know the code
better, that should be easier.


	- Eero