- Resolve the conflict between the browser behaviour in concat scroll
  snap mode and the scroll button with delays
- Add space at the bottom of each page to ensure that the user
  can reach every element

- add padding
- add stopping on every page
- change selected index recognition

- The workarounds for ff with setTimeout are no longer necessary in the
  new version and have been removed
- Replace ChangeDetectorRef.detectChanges with setTimeout because the
  scroll button was not always reliably displayed at startup

This button helps the user to detect if a page has additional,
hidden content

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

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

* Use `IntersectionObserver` to detect page position while scrolling

* Pages can be scrolled or tabbed
* Labels of pages can be shown and hidden
* Always visible pages can be left, right, top or bottom
* Layout of pages is now handled in `LayoutComponent` and removed from
`PlayerStateComponent`
* Due to the `ScrollIndexDirective`, pages with scrollbars have similar
interfaces as pages with tabs