* 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.

This does not need to be an observable as it is used only internally and 
in 1 component, which needs the new state only after calling update 
functions. Those functions can return the new value directly instead of 
via observable. And for store internal use a simple variable is 
sufficient.

The Material drag and drop stuff does not play well with the 
implementation of elements being styled dynamically via CSS. The extra 
div element pulls them apart avaoiding problems.

This was caused by an internal sorting mechanism of the drag and drop 
module.

This defines the elements priority when stacking elements on the canvas. 
Directly maps to the CSS property.

Also show tooltip on the input to explain what it is for.

To avoid any issues with changing heights.

Also keep track of already given ones.

Stronger then warning.

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

Remove unnecessary decorator and variable initializers from
`CanvasElementComponent`

Now there is a button at the side that can be used to slide the panels 
in and out.

The extra styling via panelClass needs CSS definition in local project, 
so can not be set by the Service itself but relies on local CSS rules.