* Remove workaround with `Promise.resolve().then(() => ...`
* Use `changeDetectorRef.detectChanges()` after updating the validators

Make the initialization consistent

Getter work like functions and are executed every time Angular change
detection runs.

* 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`

Replace pageIndication with translate pipe

* Add specific translation file for editor in assets/i18n
* Add first translations for toolbar

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.

- UnitService no longer knows about selection. Now needs element to be
manipulated, as param.
- Add new SelectionService handling selection of canvas elements. No
sections yet.
Move eventing stuff accordingly.
- Canvas Section now creates it's child overlay component in the
template instead of dynamic component creation. This make event handling
easier und is more the Angular Way. Also removes a lot of eventing and
selection handling from the component.
- Aligment of selected canvas elements in done in UnitService. That is
where all unit manipulation should happen.
- More takeUntil

Use `massageService` of common project.

* Move setting of validators from `FormElementComponent` to
`ValidationMessageComponent`
* Add validators and warnings to `ValidationMessageComponent`
* Move subscriptions for form controls and their validators from
`PageComponent` to `FormComponent`

This label is defined by the template below.

- use takeuntil
- pass index values instead of observables to children

* Register `PageComponent` as child formGroup, so that values for
any page can be sent to the host.
* Register formControls as controls of the page

Some elements were missing parent interfaces.
Also refactor unit factory to not return objects completely adhering to 
interfaces but records, which then can be put together to create the 
real object adhering to the interface.

Those props are set when page is created and can not be changed. This 
will have to be refactored when the functionality to switch pages is 
implemented.

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

Both use the property name 'min' and 'max' but map it to min/max and
minlength/maxlength respectively.