[BUGFIX] Render „create new content element“ wizard in modal

The "create new content element" wizard that may be triggered by using
the context menu now properly renders the wizard in a modal window.

Additionally, the patch streamlines the usage of Enum/Severity.

Resolves: #84046
Related: #75676
Releases: master
Change-Id: Id166fd5acbe19cbb969ff3d5cfef75119deb80b3
Reviewed-on: https://review.typo3.org/56520


Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>