Currency as a stackable item

This sets currency to a set id (I picked 10, as it doesn't conflict with
current flare-game data). There's still refactoring that can be done
where currency functionality was different from normal items, but this
works as-is.
Cleanup old currency-specific code in LootManager

This commit obsoletes the currency sfx and currency_range settings in
engine/loot.txt. Instead, currency loot now uses data from its item
definition.

This has the added bonus of being able to define quantity
ranges for other types of loot. For example, a single potion vs a bag of
potions.
Move currency_id setting to engine/misc.txt
Add backwards-compatible support for currency key in save files
Merge remote-tracking branch 'upstream/master' into currency
Fix items being added to full inventories
Fix some inventory sounds being played more than once
Don't allow picking up loot when dragging menu items
Merge remote-tracking branch 'upstream/master' into currency

Conflicts:
	src/GameStatePlay.cpp
	src/LootManager.cpp
	src/LootManager.h
	src/MenuInventory.cpp
Merge pull request #699 from dorkster/currency

Treat currency as a normal item