Player hand list: require init by mods

Mods will now have to initialize the list/slot in order to define the default player hand.
They may use the inventory callbacks to prevent abuse of this list.

src/player.cpp

@@ -36,7 +36,6 @@ Player::Player(const char *name, IItemDefManager *idef):
 
 	inventory.clear();
 	inventory.addList("main", PLAYER_INVENTORY_SIZE);
-	inventory.addList("hand", 1);
 	InventoryList *craft = inventory.addList("craft", 9);
 	craft->setWidth(3);
 	inventory.addList("craftpreview", 1);