[BUGFIX] Use correct backend layout in FE

Skip unqualified pages for backend layout determination.
This is basically a copy of the code already present in
BackendLayoutView::getSelectedCombinedIdentifier().

Resolves: #71086
Releases: master
Change-Id: I420a5f042e56b879f4fb222ed00c0e8f51541cfd
Reviewed-on: https://review.typo3.org/44365


Tested-by: Wolfgang Wagner <wolfgang.wagner.fn@gmail.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>