- Check if the selection refers to the content of the text component

- Remove the manipulation of the end offset
- Add the end container of the selection to the list of the selected
nodes if end offset is 0

This disables the possibility of nested markers

When the user exits selecting between paragraphs, the offset of the
end container is set to 0. In Firefox this always (in Chrome sometimes)
leads to a misinterpretation of the selection. Therefore, the offset
is manipulated.

When the user finishes selecting between paragraphs and the selection
happens from back to front, Firefox does not consider the start
container as a selected child node. Therefore, it is added to the
list of selected nodes at the beginning.

The marking service takes over the conversion to rgb notation, because
in html markup this notation is used for the background color

- Map value between unit state and element model
- Takes over methods that were previously present in element container
- Gets and holds a list for mapping drop list elements

The entire HTML is no longer saved, but only the indices and the color
of the markers

- Ignore blank pages when calculating the response progress

The status of a page is defined by the state of its elements.
Therefore, after restoring the elements, a check of the status of a
page must be triggered.

Use verona-module-metadata.json to define json meta data in html header

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

Start could be a restart, so lists containing references to elements
are cleared.

If no audios and videos are available, the query status is 'complete'

The presentation will not be completed until all audios with the
minRun property have been played.

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

Use the status of each element

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

* Use IntersectionDetector for intersectionDetection directive
* remove unnecessary input 'index' from page component

* 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

Make sure that selection start is smaller than selection end

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

Use static readonly property MARKING_TAG to define tag name of marked
text