[CLEANUP] Rename IconUtility slot class

Additionally clean some things up:

* remove unnecessary static property
* make property private
* avoid notices

Releases: #master
Resolves: #62618
Change-Id: I73c9c4c150e32b8550e959c0dfb16d2084bfa9ad
Reviewed-on: http://review.typo3.org/33708
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>