Reagent Bag

* Reagent Bags now exist. A player can have only one. The reagent bag
cannot be dropped. Any reagents acquired will automatically go into the
bag. Right clicking on the bag will show the contents, and a stack of
reagents can be pulled out for sale or conveyance. Reagents in the bag
will be found and burned by spellcasting. This should significantly
clean up our inventory space.

* Upgraded Holder class GetBulkHold and GetWeightHold to calculate their
bulk and weight whenever they are asked, rather than keeping track of a
property. Deleted the property, so all holder objects will lose
piBulk_hold and piWeight_hold.