This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.

- Refactor findNestedUIElement as static class method

Change creation of the list of possible drop list values.
The values can be found at different hierarchy levels in the unit
definition, as they can be sub elements of cloze elements

Add styles for fixed right position
Customize markup for different positioning
Refactor Usage of KeyboardService in components

Fixed-width pages are always centered in the browser, even if the
specified percentage is not equal to 50%.

The percentage weighting of the pages has an effect if the width of the
pages is greater than the width of the browser window.

This reverts commit 93e62a4a.
Teststudio does not accept verona 4 specification data

- 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