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

This includes turning off no-use-before-define completely.

* 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.

The exact type of the value is still specified in the child class.

Those have a required setting and warn message.
- Also refactor SurfaceElement to no longer inherit from UIElement as 
this is not needed since we can have multiple inheritance for 
interfaces.

Also refactor a lot of stuff belonging to this:
- You can edit the default values via properties panel
- All elements now again have a common parent directive. form elements
have a special parent for forms.

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

* Add verona interfaces to ensure type-safe communication with host
* To use the verona interfaces refactor the verona services

* Implement `VeronaSubscriptionService` to subscribe to messages from
the host of the player
* Implement `VeronaPostService` to post messages to the host of
the player
* Implement `NativeEventService` to subscribe to events like 'scroll',
'blur' and 'focus'
* Remove js code from wrapper index.html. It is no longer needed.