Td ai improvements

[giulianobelinassi]

Tiberian Dawn AI in skirmish was backported from RA, but it do not take into account many pecularities from TD. This commit introduced the following changes:

1. Sell off buildings when health is low and there is no money to repair
   them.
2. Initial build order try to be CY -> Power -> Barracks -> Ref -> Ref
   -> Anything else
3. Randomize one of the "best" buildings from the "best" vector.
4. Allow Nod to build Helipads.
5. Allow Nod to build Comm Center after airstrip for tech.
6. Reduce the maximum amount of Obelisks so the AI don't get stuck
   with low power
7. Allow Nod to build airstrips.
8. Fix insane bumps on hard difficulty.
9. Allow GDI to build Guard Towers and Adv. Guard Towers.
10. Allow GDI to build Repair Bay only after War Factory.
11. Allow GDI to build Comm center -> Adv. Guard Towers if it finds
    itself theaten by air units.
12. Reorganize sell urgency if it finds itself without money.
13. Reorganize sell urgency if it finds itself without power.

Many changes where done by Gerwin2k, TobiasKarnat and myself.

This also fixes the bug reported in #837 .