Allow dropping multiple items at once

Fixes #970

Since std::queue is now used in a good handful of files, I've added it
to CommonIncludes.h
Better handling of missing items

If the player loads a save that contains an undefined item id, we now
create a blank "Unknown Item" and add it to the items vector.

Notable changes:

* fillEquipmentSlots() has moved from MenuItemStorage to MenuInventory.
Item's that can't find a proper slot are moved to the player's normal
inventory.
* If there are no loot animations for an item, we no longer crash when
dropping the item on the ground.
Merge pull request #971 from dorkster/items

Better handling of missing items