Rewrite AI BaseBuilder and add tech levels to TD.

The main blocker for tech levels was that they exposed the extreme dumbness of our AI base building.

This rewrites that part of the AI to be more sensible (there are still a couple of hacks carried over, and this doesn't touch any of the non-building AI bogosity), and implements four tech levels for TD:

* Low (everything before comm. center).
* Medium (everything before advanced comm. / temple).
* No Powers (everything except airstrike, nuke, ion cannon).
* Unrestricted (everything).

Fixes the bulk of #5416.
The whole idea of having a single base center that everything is built around is bogus anyway.  Hopefully some future cleanups will remove the need for this.

I turned this down to win some free perf in TD and RA where 40 will never make sense, but it may make sense to move this into HackyAIInfo so that D2K can keep it at 40.