[TASK] Extract status identifier to separate property

Instead of parsing the status objects for their classes it is better to
refactor the severity status as own property. So even the CSS class
backparsing can be dropped.

Change-Id: I6d709017e4b98582aa3b64e512107999d4f2bbb2
Resolves: #46943
Releases: 6.1
Reviewed-on: https://review.typo3.org/19591
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn