This is now only one field with string value. Unset is put by using 
'none', since mat-select inputs cannot deal properly with null values.

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.

Applies to textfield and textarea.

Add styles for bar and buttons

When those switches are on the size values are ignored and calculated 
automatically instead.

This makes it easier to define types of objects in surrounding code.

This avoids import cycles.

This keeps import cycles from happening.

For now this is only relevant for checkbox values. Would be better to 
have a layer of abstraction of top. Maybe BooleanInputElement.

This makes checking the existence of properties for the property panel
possible, since it is possible to check for null but not for undefined.

to avoid name collision.

There are still some problems. It works a lot better than before. Adding 
this as an intermediate solution.

It does not support changing the fnt size.

... and no longer an Angular service.
This is because the IDService is needed in classes, where it is not 
injected by default.

- 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.

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

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

* 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.