[TASK] thumbs.php ease require/include path

Currently typo3/thumbs.php requires t3lib/thumbs.php and
t3lib/thumbs.php requires EXT:backend/Classes/View/ThumbnailView.php.

Include ThumbnailView directly in typo3/thumbs.php.
But keeps deprecation in place and other files untouched.

Change-Id: I1c464e683cb08f674ac47cb87f296b185fd35db5
Fixes: #46308
Releases: 6.1
Reviewed-on: https://review.typo3.org/18928
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn