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outdated definitions.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

- This improves scroll snap behaviour
- Set background color of page
- Interpret the margin of a page as padding. What it really is

- Refactor findNestedUIElement as static class method

Prevent sending a valueChange notification

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

- This behaves analogously to the floating marker bar

- 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.)