New AI expansion code

[forcecore]

• Select expansion place by actor (blossom tree, mine)
• Cluster them together, those actors are very close (distance < 10?)
• Exclude actors which are under AI's CY influence (build radius)
• Exclude ones within enemy CY's influence.
• Among those left, pick one that is closest to AI's CYs.
  • Sometimes, ones that are closer to enemy influence works, as an aggressive expansion, but for now, let the AI be defensive.
  • Well, aggressive expansion could be given as a boolean parameter, for modders