A small scripting language embedded into the fgd

[tmob03]

Trenchbroom currently has this, I believe it's main use there is setting models depends on the keyvalue because quake-like games don't do that automatically like source hammer does for the model keyvalue.

That ability would be nice to have here too, for example the defrag weapon spawner could change its visual model depending on the keyvalue. However what I believe is most useful would be hiding other keyvalues depending on the value of another. For example in trigger_push, it has a keyvalue called reorient_landmark that only applies when the mode is set to 4. reorient_landmark should be hidden until mode is set to 4, as it is unneeded in all other cases. Another use case would to manually set another keyvalue based on the value of another, but I can't think of a use case of this off the top of my head right now.