[BUGFIX] Let Web-View respect TCEMAIN.previewDomain

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

Change-Id: I7193cd93a2b9b2d175e162d3fab0b837aefd2487
Fixes: #61226
Related: #30889
Releases: 6.3, 6.2
Reviewed-on: http://review.typo3.org/32512
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>