Battle of Britain remake of PSS game CodeSVN

Air combat: corrected checking if combat occurs among squadrons. Now German squadrons (not groups yet) can be attacked by multiple British squadrons.

Minor adjustments and planning air combat (comments).

Groups:
- Squadron may have a leader (leading squadron) which it follows (sets the same target as leader's target)
- Leading and following squadrons may occupy the same position (allows to combine fighters with bombers)

Air combat:
- Added new icon for fighting squadrons.
- When enemy squadrons meet and british one has any ammo, combat occurs (without losses for now).

Randomizing number of planes crashed because of lack of fuel. Number of planes crashing increases geometrically with every turn spent in air "without fuel".

Minor adjustments: corrected returnPlaneType in Squadron.java. Corrected formatting of text in squadron landing menu.

Airfileds: adjusted information shown in airfield menu - added info about airfield's status (operational, closed), runway and weather. Added information about type and number of planes in landed squadrons. Corrected position of labels in airfield's menu. New methods in airfield.java and squadron.java returning string value of status, runway, weather, plane type.

Landing of british squadrons: added checking whether airfield is full or not (number of squadrons landed already must be lesser than size of the airfield to let new squadron land)

Minor change: added descriptions of values in squadrons.txt and changed code for reading this data accordingly

Added gameclock.