[BUGFIX] Static access to Enumeration constants

To get all available constants for a TYPO3\CMS\Core\Type\Enumeration
an instance of the Enumeration is required. This is changed by
defining the `getConstants` method as static and using
`get_called_class()` instead of `get_class($this)`.

Resolves: #61090
Releases: master,6.2
Change-Id: I45e8241c6f41a23a1748e620c000b6dd59d6f389
Reviewed-on: http://review.typo3.org/33244
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>