Custom design sets.

[DaloLorn]

Assemble a system to load different sets of designs (and even upgrade them if the technology is available) depending on an empire's lifestyle.

• [ ] Figure out the basic design set definition format.
• [ ] Figure out how to handle upgrades.
• [ ] Convince the design loader in bai_act_fleets to use the design set system.
• [ ] Write up design sets for all of the current races. (And all of the races' possible FTL combinations... that's going to be a hard one...)