Implemented Potion Belt and associated features

Implemented a Potion Belt which extended the bag class. 
Reordered Bag appearances in shops to Potion Belt(6), Seed Pouch(11), Scroll Holder(16), Wand Holster(21).
Adjusted tab size to fit 5 tabs as opposed to the original 4.
Implemented buff-item interaction where there is a chance that one of the hero's potions will shatter when the hero is frozen. Potion Belt will prevent potions from being shattered when hero is frozen.