[FOLLOWUP][TASK] Render backend icon list by existing files

* Re-added the "spinning" option for spinners (thx Nathan)
* Changed cache from backend_core to asset (thx @helhum)

Resolves: #82348
Releases: master
Change-Id: I1e59da79dc7072b582866213a571e35bfee4b639
Reviewed-on: https://review.typo3.org/54061


Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>