[TASK] Deprecate VERY old BackendController hooks

When the BackendController was just a plain backend.php file (back in TYPO3 6.2)
there were some hooks available to load ExtJS components and JS/CSS files
via include statements and relative paths.

There are now better hooks within BackendController, so CSS and JS resources
can be loaded via PageRenderer and within the BackendController scope.

The obsolete methods and hooks are marked as deprecated.

Resolves: #80491
Releases: master
Change-Id: I9e0e72df7237ab958ec06f7d58a9fa41c75ba890
Reviewed-on: https://review.typo3.org/52187


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>