[#60] Fusion Tab on item sheet

[krbz999]

Closes #60.

• Moves active fusions and fusion options into their own tab on the item sheet.
• For now, the tab is always visible but displays a hint for invalid item types (all types other than arsenal and armor).
• If the arsenal or armor item has the 'Fusion' attribute, they can create a fusion option on this new tab.
• If they don't have that attribute, this tab only displays any active fusions.
• A fusion option is no longer required; if omitted, a default "dummy" effect is used which has no additional changes.

Follow-up work: Split item sheets from "Base" into one per item type for better configuration.