Hovering an affix in the loot filter should display how many in stash

When we hover the mouse cursor over an affix within the loot filter, there should be a popup that displays how many of that affix we’ve got stashed.

Reason: The loot filter is used to keep track of affixes we want to farm. If we’ve already got plenty of a useful affix in our stash, then it’d be practical to be able to tell via the loot filter itself (instead of closing it to check the stash manually). This would make the filter easier to fine tune (fewer manual steps).