[FEATURE] Add option to set an alternative domain for preview

Make it possible to set an alternative domain for preview with
TCEMAIN.previewDomain = example.com
in PageTS. It is useful for multidomain sites.

Change-Id: If69115e877be310225c0e21685df210ca7973164
Resolves: #30889
Documentation: #52781
Releases: 6.2
Reviewed-on: https://review.typo3.org/24730
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind