Refactor styles for elements and element overlays to achieve stretching
of dynamic sections when text areas are resized

* undo setting overflow hidden for sections
* set display of `ElementOverflow` to block

In the past, the page index could be set by button click and after
scrolling, which led to unwanted side effects

To center the content of the player, the dimensions of all pages are
calculated.

* Move two different intersection detections from
`scroll-index.directive` and `section.component` to
`intersection-detection.directive`

* Fix bug for small pages
* Add functionality for tabbed pages
* Use layoutContainer as container for intersection detection
* Rename properties and methods