- Remove interfaces
- Add classes for all elements as well as unit, page and section
- 2 intermediate interfaces are kept. They dont constituate a viable 
object but only additional properties, like background-color. Those can 
be added as interface to all elements. Initialisation has to be done by 
the element itself though and not by some constructor higher up.

Put the possible element types explicitly.

This was an early draft and unused. Will be reintroduced in a proper 
form later.

The different tabs are now separate components, which makes them a lot 
smaller and better to handle.

This replaces the label text with an image, which can be uploaded.

The existence of props can be easier checked via null-check.

This is rather hard to implement via HTML inline style attributes. So we 
do it the classic way.
Could be reviewed, when that feature is implemented in TipTap:
https://github.com/ueberdosis/tiptap/issues/1514.

Based on TipTap which in turn is based on ProseMirror. No further assets 
are needed.

The editor package is larger than that. So the build fails without the 
higher limit.

Will be replaced by new editor module. See subsequent commits.

* When elements receive the focusin event, their status is changed to
`TOUCHED`
* Replace `this.form.valueChanges` subscription with
`this.unitStateService.unitStateElementCodeChanged` subscription

* Take over tasks of `UnitStateComponent`
* Manage property `unitStateElementCode`
* Removing the passing of the property `unitStateElementCode` by
the components

Console logging is disabled in production mode by overriding the
function in `main.ts`

* Implement as draggable overlay
* Keyboard can be configured as 'mini' or 'full'

* Value and status of all elements are saved
* Move some Methods and Properties from FormService to UnitStateService

* `UnitState.dataparts` has property `elementCodes` property, which
stores status, value and id for FormElement
* When building a unit, the stored value is assigned to the element
when it is initiated
* Removed `UnitState.dataparts.pages`
* Removed methods initPages and addStoredValues

The DOM query is now only done once and nt again everytime the value 
changes.

* Rename service to `KeyboardService`
* Replace the two fixed rows for upper and lower case letters
of the keyboard with a general row array
* Rename css classes, variables and methods

* Adapts their appearance to mat-form-fields
* Fix coloring of background for checkboxes

Overridden by some child elements (TextField, CheckBox)