[BUGFIX] Add property icon to category domainmodel

Add missing icon property

Resolves: #53341
Releases: 6.2
Change-Id: Ib84e60c59a46d6f19adc1426760f177a37ddfe61
Reviewed-on: https://review.typo3.org/25170
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer