HaikuDepot: LRU Cache for Icons

Only keep a fixed number of icons in memory at once.

Completes To #15370

Change-Id: I23e3a4fa7559894034f45afb3b536910ea037078
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3367
Reviewed-by: Rene Gollent <rene@gollent.com>