- Using scrollIntoView with target 'center' was not always reliable
  especially in FF

SectionVisibilityHandlingDirective now controls the visibility of
sections that depend on an audio or video

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Change import paths of CommonModule files in editor and player

to ensure that a section that is not displayed has no dimensions

Sections use the mediaStatusChanged event, which is also used to map the
interdependence of media elements

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

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.

Use the status of each element

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

* Remove FormService Injection
* Move FormService and form types to player
* Move interface ValueChangeElement to uI-element.ts
* Use interface InputElementValue instead of
string | number | boolean | null

* Take over tasks of `UnitStateComponent`
* Manage property `unitStateElementCode`
* Removing the passing of the property `unitStateElementCode` by
the components

* `UnitState.dataparts` has property `elementCodes` property, which
stores status, value and id for FormElement
* When building a unit, the stored value is assigned to the element
when it is initiated
* Removed `UnitState.dataparts.pages`
* Removed methods initPages and addStoredValues

* 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

Required to decide whether the `section` or `element` should be inserted
or appended to its `formArray`

* Since always visible pages are removed from the order, form
arrays must be rearranged
* Refactor getter as properties in `PlayerStateComponent`. Getter
work like functions and are executed every time Angular change
detection runs.

* Create `FormGroups` without ids
* Add `FormArray` for elements

* Add section array to the form groups of the pages
* Remove unnecessary component factory from `SectionComponent`

* 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

* The wrapper of the player component listens to the `valueChanged`
event and posts a message to the host of the aspect-player
* Improve code style and remove lint errors

Use `parentForm` input in `PageComponent` and in `SectionComponent`
instead

* Use all style attributes of the unit definition to display
sections
* Remove unnecessary div tag from `SectionComponent`