[BUGFIX] iconIdentifier for custom system information panel items

An iconIdentifier is needed since the icon generation is
moved to the Fluid template. The third parameter of
addSystemInformation() needs to be an iconIdentifier
instead of an rendered icon html markup.

Resolves: #81694
Related: #80554
Releases: master, 8.7
Change-Id: Ib6b07281196b5ecda2de07b8d6328b524e08f038
Reviewed-on: https://review.typo3.org/53321


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>