[BUGFIX] Let Web-View respect TCEMAIN.previewDomain

Added previewDomain in order to have Web-View functioning in the
same way like Page-Preview does.

Fixes: #61226
Related: #30889
Releases: 6.3, 6.2
Change-Id: I872651aeac1bda27f7a8a034fc30a21ff7162df0
Reviewed-on: http://review.typo3.org/32510
Reviewed-by: Oliver Gassner <og@plan2.net>
Tested-by: Oliver Gassner <og@plan2.net>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>