[FEATURE] Optional label attributes, automated LLL reference building

Added automatic LLL insertion to make the `label` attribute on the mentioned ViewHelpers optional. If label is not added, special LLL references will be built instead (enabling the debug feature provides verbose information).

Some ground rules:

* Works only on Flux forms registered with an appropriate extension key relation, which can be achieved in three ways: 1) Use the extension key as index when adding the collection paths. 1) When adding TypoScript paths for your collection, add an "extensionKey" attribute next to the path definitions (useful if you can't or won't use the extension key as index). 3) Once registration through PHP becomes possible, the relation is made whenever PHP registration is used.
* Only generates an LLL reference if a label was not specified on each individual entity.
* Prefixes every reference in the TCE array with `LLL:EXT:extensionkey/Resources/Private/Language/locallang.xml:flux.` to isolate automated labels. Should automatically work with XLIFF thanks to to internals in TYPO3.
* Displays a completely empty label if no LLL value was found for the current language - this is expected TYPO3 behaviour but can be hard to investigate.

Example automated labels:

* …:flux.objects.fluxFormId.objectName
* …:flux.objects.fluxFormId.objectName.fieldNameInObject
* …:flux.flexforms.fluxFormId
* …:flux.sections.fluxFormId.sectionName
* …:flux.fields.fluxFormId.fieldNameNotInObject
* …:flux.areas.fluxFormId.contentAreaName