[CLEANUP] Streamline backend toolbar items

- Move everything to Fluid (also icon generation and labels)
- Remove a lot of get* shorthand protected methods
- Add PHP comments and remove PHP comments where applicable

Resolves: #80554
Releases: master
Change-Id: I9c417a02b2ac41fee95b815ee626ff44523c96f5
Reviewed-on: https://review.typo3.org/51909


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>