[BUGFIX] Fix impexp with images in RTE and links to files

This patch fixes the import and export of RTE contents
including images and links to files as well as softreference
links to files (file:{uid}) and adds functional tests
for this use cases.

Resolves: #56089
Resolves: #56285
Related: #56286
Releases: 6.2
Change-Id: I29312b304d265581bf9f35bdc2692a19d2df5a93
Reviewed-on: https://review.typo3.org/28105
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring