- Remove element as parameter

Compound elements are now supposed to use overlays for their child
elements. This overlay makes selecting child components (by clicking on
them) and marking them as selected (done by the selection service)
possible.
The SelectionService no longer needs special logic to handle compound
children selection, as they now also have an overlay with the same
interface as normal (canvas) elements.

A few modifications in connected directives are necessary. The now
handle children components in a proper array instead of QueryList.

Likert elements do not have clickable children yet and work a little
differently. This should probably be unitized in the future.

- Implement marking buttons as toggle buttons

Do not consider all form elements, but use a list containing only form
elements with validators

Refactor MathKeyboardComponent to reduce code size

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.

- Improve variable naming for unit definiton type. (version string)
- Use 'iqb-aspect-definition@1.0.0' as value which better communicates
  the purpose of the data.
- Also update the HTML files with the new definition. (Even though it is
  not checked anywhere.)

For compound elements, the form has more than one control in its
element array. These must all be checked for their valid state.

- Explicitly set the value of the form control of CompoundChildren,
because they are not yet fully registered in the onInit phase.
- Use ChangeDetectorRef to prevent
ExpressionChangedAfterItHasBeenCheckedError

See 5ff73e14

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

- Do not check whether pages or sections are valid
- Check the entire form and if necessary the controls

- 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

When a page has longer scrollable dynamic content that consists of many
individual elements (Likert), the content was sometimes not displayed
after interacting with those elements. This happened in the maximized
browser display, especially in Firefox.
Probably due to the deep dynamic nesting the height to be displayed
could not be calculated correctly

Define an offset for textarea or textfield during initialization.
This prevents error messages from the linter.

Show warning messages of input fields at event 'NavigationDenied' only
if reason "responsesIncomplete" is sent

- 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

This prevents the section from being placed in front of elements
with negative z-index

This creates a blockquote-element, which is styled globally with 
specific quotes and alignment.