[FEATURE] Allow import of XML structure

A new property "xmlValue" is introduced which allows for
preserving the XML structure of child nodes of the imported
nodes.

Resolves: #63012
Releases: 2.4
Change-Id: Ie1214cf14f539e250b5785dad3e3032e7fff9d13
Reviewed-on: http://review.typo3.org/34254
Reviewed-by: Francois Suter <francois@typo3.org>
Tested-by: Francois Suter <francois@typo3.org>