* 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`

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

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.

* 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

* 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`

* 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

Remove unnecessary decorator and variable initializers from
`CanvasElementComponent`

Shift the order of style assignments in `updateStyle` method

This is part of trying to solve the problem of upating styles on already 
rendered components, but still does not work.

THis makes the  components usable in other contexts (player) as well.

This makes the actual component elements agnostic to any drag and drop 
and selection logic, which is handled completely via the 
wrapper-overlay. Now the component elements can be easily used by the 
player.

They will be reused by the player.

There it can be used by the player as well.