Implemented qrw::Path::getMovementCosts().
qrw::Engine now uses pathfinding mechanics to calculate movement distances.
Implemented qrw::Engine::findPath().

This is just a wrapper for the intern used AbstractAlgorithm for pathfinding.
Added basic qrw::Path handling in qrw::BoardWidget.
Fixed bug in qrw::Engine that prevented units from attacking.

This occured because the path found via pathfinding was 0 if the destination square was occupied by an enemy soldier. The engine only checked if path != 0 so it was never possible to attack an enemy.