[TASK] Initial refactoring of toolbar items

* Introduce new (WIP) interface ToolbarItemInterface
* Adapt current toolbar items
* Toolbar item html scaffolding is done by BackendController

Change-Id: I237792bf3277fcbc87762014d21ec655603e150f
Resolves: #62929
Releases: master
Reviewed-on: http://review.typo3.org/34034


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>