Calculate once, update when needed

[Pierre-Sassoulas]

Right now calculations are done very often in particular for ArkInventory and Bagnon (for each update of a ButtonFrame). The addon should calculate once use a global value and then re-calculate only when needed. (In case of loot, exchange, sell, or throw ?)

[Pierre-Sassoulas]

Also need to take change of configuration into account, or adding an item in the peddler/scrap list.