[TASK] Clean up workspace preview top bar

The preview controller is now not attached to the backend module anymore,
as the Preview functionality ("Split Bar on top") is technically a completely
separate backend preview and completely unrelated to the workspaces
Backend Module.

This change also removes several hacks to generate Backend URLs.

Resolves: #84074
Releases: master
Change-Id: Id0ce7093f6d3ac94c8e4a9cfecd78027d26d45df
Reviewed-on: https://review.typo3.org/55931


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Gorges <daniel.gorges@b13.de>
Tested-by: Daniel Gorges <daniel.gorges@b13.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>