[TASK] Allow preview generation to be controlled from Provider

Although technically also a bug fix (unexpected preview content if Provider detected for tt_content, due to missing $fieldName which would have filtered the Provider resolve operation) this is simply an added third return parameter from getPreview() on Providers, this third parameter when set to FALSE will stop rendering of the preview. The default value used by all Providers unless getPreview() is overridden, is TRUE (meaning "allow preview to be rendered by internal TYPO3 methods").