XCom-like cover system

Adds a cover system like the ones in the XCom reboot When a unit stands near a tile that offers dodge bonuses, it will "take cover" behind the tile. It will get those dodge bonuses as long as the cover stands between the attacker and the defender. Known issues: -this only works for avoid, i will update it in the future to give defensive bonuses too -there's no AI change, so the enemy units won't be able to fully take advantage of the system. they seem to understand where they get more hit and avoid tho. if anyone could help with this, it would be welcome! I actually don't know javascript and write scripts by kinda copying what's already in, so no guarantees ;) -if the attacker stands on the cover, the defender will still take the avoid bonus. cover is supposed to be impassable like in the xcom games, but i may try fix it in the future Small update: now you can "peek from cover" like in the xcom games: if you are behind cover and you could get a line of shot by "peeking" from the cover, you'll negate the avoid bonuses!