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)

* After deleting validation component, element component is no
longer needed and is deleted
* Element overlay takes over the methods of element component

* Delete validation message component
* Use instead `mat-error` inside element components to take advantage
 of `mat-input-field`
* Add get validators method to each form element component
* Display validation messages via `error-transform` pipe

Accidental closing of the keyboard can happen when the user clicks on
the board under the keys

* Bugfix for units without always visible page
* Bugfix for aspect ratio over 50%
* Integration of keyboard

Since the material elements don't give a way of applying the background 
color, a hack is needed to get the internal elements and apply the 
style.

Directive to be able to do it for text field and text area.