This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

This prevents the browser from trying to translate the page on
German systems

Move required styles and classes to the overlays of player and editor

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outdated definitions.

Remove the wrapper divs in each element component and add the
'fixed-size-content-wrapper' class to the element overlays
of player and editor

The previous solution, which uses a top position of 50%, did not work on
iPad

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