Since the XEE security fix (I26701fc2ffb5aed7ccbd96c168aef571d012091e),
the XML files in the t3editor are not loaded anymore. Change the way how
the files are loaded to fix the issue.

Change-Id: I26c622e47ee0f791b998886837f4443f5bddf11b
Resolves: #74508
Releases: master, 7.6, 6.2
Reviewed-on: https://review.typo3.org/47173


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>