Larger inventory

The inventory should be increased from 36 slots (3 rows of 9 plus the hot bar) to 63 slots (6 rows of 9 plus the hot bar). These extra rows can be locked behind achievements or beating bosses. 1 row of 9 slots given as a reward for going to the end. 1 row of 9 slots given as a reward for beating the ender dragon. 1 row of 9 slots given as a reward for beating the wither.