Added Player active status.
Engine now updates Player::active states.
Added comments for Player::set/getActive().
Renamed Player::getActive() to isActive().
Board holds now separate pathfinding algorithm.
AStar: removed check for unit on start coordinates.

No longer checking if the start square is occupied by a unit, because this has nothing to do with the pathfinding.
Unit: Added getter and setter for Square.
Unit takes the board as mandatory constructor argument.
Fixed Player::setActive().
Added Unit::canMoveTo().
Added unit::moveTo().
Unit::canMoveTo() now returns Path instance.
Fixed movement cost handling in Unit::canMoveTo() and Unit::moveTo().
Engine now sets Square to Units.
Added struct BattleResult.
Added unit::canAttack().
Made Unit::getPlayer() const.
Distinguishe between base and modified attack/defense of Unit.

Attention: this commit breaks compilation!
Unit::setHP() prevents HP falling below zero.
Refactored Unit::attack().