ItemTable refactoring

[flend]

Adds a power member to itemTable. Uses the power member to remove some of the abuses of strengthRequired (e.g. meaning nutrition for food or 'what bolt is it?' for wands and staves). This power member is used for effects that must be deterministic (e.g. to support the 'are you hungry enough' check for food'. Uses the power member to allow enchantment scrolls to have a multiple effect (used in RB due to the low number of levels and not wanting to be flooded with scrolls). Replaces magic numbers in potion effects with use of the (existing) range member, as per how is done for armour.