refactor HexdameGame

split the grid into its own class
implement copy constructor/operator and ==
use empty move instead of invalid Coords
move validMoves to HexdameGrid and return const &
works just as well
don't allow dragging pawns that can't move
allow iterating with foreach over the grid
working negamax
implement alpha/beta pruning
reduce the value of kings
copy validMoves when copying grid
add zobrist keys, switch to QVector when saving grid