[TASK] Move typo3/stylesheets to EXT: backend

In order to decouple extensions from each other and to
fasten bonding between files to specific extensions the
folder typo3/stylesheets should be moved to EXT: backend.

Change-Id: I2ecf68e9c8d412c1c9b097db63743bde518f52b0
Resolves: #49747
Releases: 6.2
Reviewed-on: https://review.typo3.org/22048
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert