[TASK] Make IconRegistry::getAllRegisteredIconIdentifiers() public API

The method IconRegistry::getAllRegisteredIconIdentifiers() was introduced
as a public function with an internal scope, but there is not reason to keep
this useful method "private" anymore.

Resolves: #77691
Related: #69054
Releases: master, 7.6
Change-Id: Ic98cc1f6a1f44926b6fd64a05b3678dcd638ddbf
Reviewed-on: https://review.typo3.org/49649


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>