Rename KeyboardComponent to KeypadComponent
Rename KeyboardService to KeypadService
Split LayoutComponent into LayoutPlayerComponent and
LayoutPagesComponent
Prepare the introduction of an additional keyboard

* Bugfix for units without always visible page
* Bugfix for aspect ratio over 50%
* Integration of keyboard

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`

* 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