Allow record sliding in page.resources.fal

Page\Resources\FalViewHelper was until now missing the feature to use record sliding for page-related media. That is however commonly required for dynamic page background images and such and I wanted to avoid falling back to TypoScript.

It made quite a challenge to separate the already existing sliding code from AbstractContentViewHelper into a separate class and I'm not sure whether the result is quite right. If you have suggestions concerning the implementation of this feature, I'd be happy to rework it.
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