- Reimplement UnitStateComponent as central point to send the collected
information
- Implement ValidationService to determine the responseStatus of all
forms

Remove manipulations from elementModel
- Refactor drop list component
- Use of initial values for audio and video playback time

Defining the validators not in the elements anymore, but when creating
the forms

This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

- This behaves analogously to the floating marker bar

- Rename NumbersKeyboardComponent to MathKeyboardComponent
- Decrease the number of input properties
- Split source code into private methods

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

* Remove general (full) keyboard in layout component
* Implement French keyboard component

to avoid name collision.

* Implement as draggable overlay
* Keyboard can be configured as 'mini' or 'full'

* After deleting validation component, element component is no
longer needed and is deleted
* Element overlay takes over the methods of element component

* Delete validation message component
* Use instead `mat-error` inside element components to take advantage
 of `mat-input-field`
* Add get validators method to each form element component
* Display validation messages via `error-transform` pipe

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

* 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

Move `MatDialogModule` from editor to common
Add `AlertDialog` to player

When elements are dynamically positioned there is no space below the elements

* 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

Refactor some translations

more information about [ngx-translate](https://github.com/ngx-translate/core)

In addition to the scrollable pages, units can also have one page that
is always visible.

`PlayerStateComponent` encapsulates information about the status of the
player.

Move specific form functionality from `AppComponent` to a new
`FormComponent` to make the code cleaner

* When creating components for each element a
`ValidationMessageComponent` is also generated dynamically
* Prepare usage of default values for form elements
* Add `form.ts` for form specific interfaces
* Rename some variables and properties

Move all player components to component folder but `AppComponent`
Improve naming of variables

- Replace in the projects 'editor' and 'player' the selector
'app-root' with 'editor-aspect' or 'player-aspect' and make a
case distinction unnecessary
- Remove definitions of the property 'local' in `environment`
and `angular.json`