[TASK] Add test case for Page\Resources\FalViewHelper
[FEATURE] Introduce AbstractSlideViewHelper

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.

AbstractSlideViewHelper implements all functionality commonly required for sliding and should thus be used as base class for all view helpers that, optionally, want to support sliding, even if only in a subclass.

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.
* $slideCollectReverse now selects the correct page range from the rootline when setting it to a value greater than zero.
* $slideCollectReverse can now be set to -1 and used without also setting $slide to -1.
[FEATURE] Make AbstractRecordResourceViewHelper inherit from AbstractSlideViewHelper

This is required for using record sliding in Page\Resources\FalViewHelper which is a subclass of AbstractRecordResourceViewHelper.
[FEATURE] Make Page\Resources\FalViewHelper support record sliding

This is achieved by utilizing the new functionality introduced by AbstractSlideViewHelper. Custom ordering of the records is not supported, but probably also not needed.
[TASK] Make AbstractContentViewHelper inherit from AbstractSlideViewHelper

The sliding-related code from AbstractContentViewHelper was adopted into AbstractSlideViewHelper and thus does not need to be duplicated here.
[FEATURE] Adopt Content\Random\GetViewHelper to correctly support record 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.