[CLEANUP] Cleanup backend toolbar item registration

* Remove side effects from class file
* Move registration script to Resources/PHP folder

Resolves: #61467
Releases: 6.3
Change-Id: I0786a1d2bddefac17780168acbd2e088c2e5690a
Reviewed-on: http://review.typo3.org/32668
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>