Export additional unit property info.

There's a lot of additional data available in the lua database that
informs any callers how to implement the UI. Expose that in the unit
type as well.

I've added this as a new API surface rather than extending the data in
the old API which is only queryable via reflection. The old API is good
for hard-coded mission generators that want to set properties
explicitly, but for anyone trying to create a UI for setting these (such
as Liberation does), it's preferable to have access without reflection.