Add icons to the Bags currency dropdown

[andybergon]

What does this PR do?

Show currency icons beside the names in Bags > Enabled Currencies, including retired currencies when a texture is available. Uses the dropdown's existing icon support; search, grouping, and tracking behavior stay unchanged.

How was it tested?

• Installed the two-line change on live with a verified backup. The supplied after screenshot confirms icons render in the currency dropdown.
• git diff --check passed. Retired currencies and missing textures were checked in code, not verified in game.

Screenshots

Before:

After:

Checklist

• New settings default OFF: no setting added; this visual enhancement appears whenever the dropdown is opened.
• Zero cost while disabled: no new events, polling, or hooks; icons use the existing menu row construction.
• Cheap while enabled: no new polling, timers, or per-frame logic.
• No new writes onto Blizzard-owned frames.
• Tested in-game on live via the supplied screenshot; no version gates or pre-Midnight APIs added.