[TD] Skirmish RA backported AI does not take TD peculiarities into account

[giulianobelinassi]

There are a few problems with TD skirmish AI that I noticed:

1. Nod AI, it never builds an airstrip nor helipads.
2. Nod AI runs out of power very quickly because it spams too much obelisks
3. GDI AI doesn't build AGTs.

There are also a few problems with the build order. I think the AI could do a more pro-like build order (CY - Power - Barracks - Ref - Ref -[Ref, War Factory]), and maybe boost the cash per harvester and build speed a little on hard mode, instead of handicaping the player.

[giulianobelinassi]

Most likely the problem is in HouseClass::AI_Building. There is no fallback to STRUCT_AIRSTRIP if it fails to build an STRUCT_WAR, and it always build an obelisk if it can, falling back to turret. There is no logic for AGT nor Guard Tower.

[giulianobelinassi]

Furthermore, the HARD difficulty bumps the enemy armor too much that anything is almost undestroyable.

[Gerwin2k]

See these VanillaTD skirmish adjustments by TobiasKarnat:
#224

(I always use these, in slightly modified form, for my local VanillaTD build.)