[BUGFIX] Streamline iframe namings for new backend

The name of the iframe is now available via top.list_frame
and top.nav_frame instead of top.content.list_frame and
top.content.nav_frame.

All tests are adapted as well.

Resolves: #78323
Releases: master
Change-Id: I6cfdafec410e45bd97cee7849fa47c1ecab814b4
Reviewed-on: https://review.typo3.org/50255


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>