Save/loading items completed

Sorry, turns out I needed that item ID after all. Added it back into the
formatting and function.
Also needed room IDs, so I added something to keep track of that.
Use WorldItems::SaveItems() to save the positions of all items.
Use WorldItems::SpawnItems() to spawn in all items (creates them).
Use WorldItems::LoadItems() to load the positions of all items from a
save file.
Use WorldItems::PlaceItems() to place items where they should be