Add flexibility to the AI for campaign/scenario maps

[wraitii]

With single player campaigns a possibility (see #4387), the AI should be improved to support a few new features, namely:

-Allow sending commands to the AI that it must/can respond to. This would allow a campaign map where you order the AI to attack with X units a target Y, and the AI would automatically deal with issues (embarking…).

-Allow disabling/enabling features of the AI at "runtime". IE you can say to the AI "don't build anything" for a while/an entire game. I believe this should be done by revamping the config system, so that it's easy to use and possibly leads to more customization in general (see #2550)

-In the same vein, it would be great if you could dynamically change the resource gathering bonus. You may want to put it at "0" so the AI still acts like a normal player but can't actually expand beyond what you've planned.

Any other thing I may have missed?

[elexis]

Backlogging due to lack of progress