[TASK] Allow xmlns declaration on fluid elements (Revision 3)

Ignore XML attributes beginning with "xmlns:", so that one can
define XML namespace declarations directly on fluid tags.

Original change by Christian Weiske for TYPO3 CMS.

Change-Id: Ic8f244413cef812bc98991078464eb93ab1662d2
Releases: master
Related: #56481
Revision-Id: e5eee7efc4aa0060018f23756ef7108cadc14a51