Various changes.

Fleshed out item code, added test item (no inventory screen yet to equip
it) which is placed randomly in the world, added an entity for dropped
items (currently only allows 1 per square, boss fights need a chest
entity), a tile may only have 1 entity at a time, controls now use the
numpad with numlock on (4/5/6/8 move, 7 use door, 9 inventory).