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.

Was an earlier try to get the correct cursor on canvas elements. This 
has been solved another way.

- Service is used by the editor exclusively so definition in
local app.module.

- Edit dialogs triggered via double click on drag overlay
- Unit Service now defines a default edit dialog for elements. For now
label or text if available.

Somewhat messy because of more nested droplists and a handler triggering 
every pixel. But the best I can do for now. May revisit later.

* 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

Use `elementModel.label` instead of `elementModel.text`

Not elements can only be dragged when selected.

One page can have this setting and is supposed to be shown at all times 
on the left side.

* Only elements which has to be registered by the player's form inherit
from `FormElementComponent`
* Element components use their specific `elementModel`
* Rename file for `FormElementComponent`

Since this does not label anything but instead is meant for (longer)
texts.

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

The UIComponents now only have the properties they can make use of. This 
means that the angular components use the CSS properties explicitly 
instead of having all kinds of useless CSS properties.
Also it is no longer needed to update the CSS styles via event because 
Angular's change detection takes care of that.

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

* Introduce `FormService` to provide communication between
element components and form
* Add `ngModelChange` and `formControl` to
some element components. This functionality is not used by
the editor

Use selected page index for similar functionality.

Also add section prop editing.