[WIP][FEATURE] Use FlexForms as addional content element configuration

The FlexForm autoloader loader, register the FlexForms of
Configuration/FlexForms/Content/ to the right content element.

The general default Controller also assign the settings to the
content element view.

Resolves: #63024
Change-Id: Iba59f26b7b8224d9e25abeee9fe560e52ba847d6
Reviewed-on: http://review.typo3.org/34298
Reviewed-by: Tim Lochmüller <tim@fruit-lab.de>
Tested-by: Tim Lochmüller <tim@fruit-lab.de>