Enemy expansion event

[orzelek]

I'd like to have an event that would be called when biters are about to expand to a new place.
It would contain location of where the expansion is going to be and allow a return value to signify that it was handled by the event and default one should be skipped.
This would allow mods to control whats built when expansion happens - mainly to make it a bit more sensible or use some schema for expansion.