[BUGFIX] Load XML files of t3editor properly

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>