[BUGFIX] Streamline backend module menu and viewport initialization

ModuleMenu and Viewport initialization need to be defered to ensure
that DOM is available in order to apply further behavior in mentioned
client implementation.

Resolves: #80312
Releases: master, 8.7
Change-Id: I9581a7f2164050f73683928b8f2ebe1d69a6d5e7
Reviewed-on: https://review.typo3.org/57526


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: Jigal van Hemert <jigal.van.hemert@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>