Improve how AI deals with shroud and hidden units

[mattsc]

This is a feature request extracted from #1035. The part of that issue that was a bug is fixed. That is, hidden enemies are ignored by the AI, independent of whether this is due to fog, shroud or the hides ability.

The remainder of #1035 makes two related suggestion for potential AI improvements:

1. Remember where units have previously been visible
2. Do not give the AI knowledge of terrain hidden under shroud

These are feature requests, not bugs.

Developing an Exploration Micro AI is on my long-term task list. It's not likely to happen anytime soon, but it would take care of both these points.