#95 Spotter planes should obey visibility

[Markus Armbruster]

Aerial spotting is a hodgepodge of rules, including:

* Spy planes can spot ships always, and land units
always, except below 20%, when there's a spotting chance.

* Planes can't spot planes (except when fighting or
bombing them, but that's another matter).

* ASW planes can spot submarines depending (random
chance related to hit chance).

Lookout is relatively simpler: it uses the subject's
spy rating and the object's visibility to determine
whether the subject can see the object.

I suggest to make the plane spotting rules more similar
to lookout. In particular, low visibility objects
should be harder to spot.