[TASK] Move preview setting evaluation in separate method

The TSFE method "determineId" is hard to read, mostly due to
the fact that it first evaluates preview settings from admin panel
or logged-in backend users, and then calls "fetch_the_id()" possibly
multiple times.

In order to increase readability, the preview settings evaluation
is moved into a custom protected method within TSFE.

Resolves: #84056
Releases: master
Change-Id: I9611801b77ab10f0e9407f2c558cbec1867cc560
Reviewed-on: https://review.typo3.org/55913


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-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: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>