New Cover System

This update changes the cover system completely.  Cover does not provide immunity from attacks anymore but simply reduces the probability of getting hit.

• units in cover can be targeted with a lowered hit probability
• units in cover crouch now
• units can shoot around corners of their own cover now
• added a system that can be used for temporary buffs
• end turn adds a 25% evasion bonus to the unit
• improved performance for unit UI