Separate powerups from items in the guide

Separate powerups from items in the guide

Since both the Powerups and Items sections use the ITEM registry, the REGISTRY_SOURCE macro had to be updated, so some minor refactoring is included.
Choose to use the ka icon for Buffs since it's more circular (like the buffs simpleitems), and dom for Powerups since it's more triangular (like most powerups simpleitems).