Display the tooltip additions for relevant items in the vendor UI

[Duckwhale]

Example: Salvage crates (from island expeditions)

It might be useful to see the progress here, but the tooltip is only shown once a box is in the inventory.

Fixing this is easy in theory:

-- Tooltip additions for units
-- (snip)

-- TODO: Display additions for all relevant items here
local function onTooltipSetItem(tooltip, data)
	print("Item ID: " .. data.id)
	-- Add attempts for item here, if tracked/enabled/not found etc.
end
_G.TooltipDataProcessor.AddTooltipPostCall(_G.Enum.TooltipDataType.Unit, onTooltipSetUnit) -- Existing tooltip hook
_G.TooltipDataProcessor.AddTooltipPostCall(_G.Enum.TooltipDataType.Item, onTooltipSetItem)

The primary issue is that the tooltip code is a bit messy and also untested, so just copy/pasting won't do.

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Split off from #601 since it's out of scope (needs some minor refactoring, probably).