Record sliding was traditionally handled solely by content view helpers. It is however desirable to extend sliding support to a greater range of view helpers, most prominently Page\Resources\FalViewHelper.

SlideViewHelperTrait implements all functionality commonly required for sliding.

Compared to the original implementation in Content\RenderViewHelper, a few changes were introduced:
* $limit is now referring to the total number of records, not the number of records rendered per page. This is important when using $slideCollect.
* $slideCollect in combination with $slideCollectReverse now selects the correct page range from the rootline when setting it to a value greater than zero.
* $slideCollectReverse has no effect when $slideCollect is zero.

This is achieved by utilizing the new functionality introduced by SlideViewHelperTrait. Custom ordering of the records is not supported, but probably also not needed.

The sliding-related code from AbstractContentViewHelper was adopted into SlideViewHelperTrait and thus does not need to be duplicated here.

…cord sliding

Content sliding was already supported, but lacked some features. Now it is possible to use $slideCollect and render more than one randomly selected content element by increasing $limit. A change in the inheritance hierarchy was necessary, but should not have any impact on the behaviour.