(v1.4.0-pre) Item sizing system overhaul

Rewritten the method for handling item sizes. Items types can now have a
slot value of any number which represents the size in arbitrary "slot"
units.

Updated the Inventory and Container modules to reflect this change and
handle new item sizes with inventory/container capacities.

Rewritten the InventoryDialog and ContainerDialog list generating and
selection code to support the new item sizing system.

Added WillItemTypeFitInInventory.