Battle of Britain remake of PSS game CodeSVN

Added methods to check pixel color on map and to calculate range between given points.

Added destroying planes while landing. Probability: 1% when runway not damaged and weather fine. +6% when runway damaged. +1% when showers. +3% when heavy rain. (probability from varius reasons accumulates)

Fixed problem with collision detection of RAF squadrons (squadrons with the same group number, i.e. 0, could have collided).

Added map icons of airfields (damaged, cratered runway, when in fog, when in heavy rain) and of radar stations (damaged, non operational)

Cosmetic changes.

Landing and takeoff delays: probably working now. Can't be sure as it's a bit hard to test ;-)

Changed: doesn't allow to land on airfield with cratered runway as runways are repaired only at the end of the day.
Allows to choose airfield for landing when it's covered in fog or runway is damaged but gives a warning (info added on landing button in menu).

Added delay while taking off in bad weather or from damaged airfield.

Starting squadrons while airfield is closed - is not allowed now.
Also squadrons' list doesn't refresh if no changes to squadrons has been made (no take offs, no landings, no refueling nor rearming).

Airfield status changes according to weather (operational/closed)