Colors and font stuff.

Saving the actual drop down value did not work with duplicate option 
values.

This makes a small (suffix) button appear which clears all the input of 
that field.

Add styles for bar and buttons

- 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.

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

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

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

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

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

Overridden by some child elements (TextField, CheckBox)

* 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

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.

* Remove unnecessary styles
* Refactor case distinctions
* Improve styles of some elements

Refactor styles for elements and element overlays to achieve stretching
of dynamic sections when text areas are resized