When this is set the page will only be rendered to that width. Otherwise 
100% of the available space is used.

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.

* 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