The MarkingService now takes care of the selection and its validation.
The text component sends its change via elementValueChanged.

All elements now use the elementValueChanged event. Also
formValueChanged is changed.

Since the options of the DropListComponent are now managed as values,
the DropListComponent can be treated as a standard FormElementComponent

Although the DropListElement is a FormElement, its value is represented
by the field options. Therefore, this state must be saved.

Use the status of each element

* Add magnifier properties to `ImageElement`
* Set used value at `UnitStateService`
* Bypass parsing error of distpacker

* Refactor markable text
* Prevent multiple selections (feature is only supported in Firefox)

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

* Remove FormService Injection
* Move FormService and form types to player
* Move interface ValueChangeElement to uI-element.ts
* Use interface InputElementValue instead of
string | number | boolean | null

* Add `InputElementValue` as type when needed

Should at least be in util folder. And since it is imilar functionality 
to the unit factory it is placed within.

Make sure that selection start is smaller than selection end

* Remove general (full) keyboard in layout component
* Implement French keyboard component

* 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

* 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

* 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

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

* 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

Break up the code into smaller methods