WIP: [BUGFIX] Make "clear value" wizard work in TYPO3 v8+

[cweiske]

The "clear value" field wizard that can be enabled with the "clear="1" attribute on "flux:field.*" elements is broken in TYPO3 v8 and later, because the registration of field wizards changed: https://docs.typo3.org/c/typo3/cms-core/8.7/en-us/Changelog/8.6/Deprecation-79440-TcaChanges.html

We register a "field wizard" that renders a checkbox that allows removing the value instead of simply writing an empty string into flexform.

WHAT DOES NOT WORK YET: Actually removing the field from flexform, because AbstractProvider::postProcessRecord does not get called anymore since commit e9127768d7b64681becbf1fb7b3774cf21bb52f3 ("[FEATURE] Use colPos instead of tx_flux_column")

Resolves: https://github.com/FluidTYPO3/flux/issues/1894

:warning: I have no idea how AbstractProvider::postProcessRecord shall be merged into TceMain.php, so I'm leaving this PR as "Work in progress" open for others to continue.