AttributeModifier and Manager added.

Allows modifiers to be defined per player and per actor.
Modifiers can define prerequisites.
Currently supports production and income.

Allows for tie-in from HackyAI. InsaneAI for RA included.

Pre-work for #6103.
This would be more clear and less cluttered as:
```
if (self.Owner != null)
{
 // code
}
```