The height of an empty droplist can't be calculated on ios 14
because it's in a flex container

Most importantly dynamic sections are now default.

Cloze documents now use this correctly.

#233

#229

- Fixed dynamicPositioning property of all section children. This 
property was accidentally set with faulty values in the last version.
- Also sanitizes likert and likert row elements, because rows now have a 
TextImageLabel instead of plain text.

- Use `inputmode` 'none' to hide the virtual keyboard of the os
- Use DeviceService to detect if the player is running
on a mobile device

verticalButtonAlignment is the new property for likert rows, which tells 
the actual radio button to either be at the top or in the middle.

Also rename LikertColumn to ColumnHeader, since it is not exclusively 
used by likert.

The LikertColumn is actually just an image-text-combo. Has a generalized 
name now.
The other interface was not needed anymore and has been deleted.

- A new new editor component has been created for this, which has a
reduced interface and produces span-elements instead of paragraphs.
- Since the created HTML (span with CSS) is regarded as potentially
harmful it needs to be sanitized. For element components the sanitizer
is put in the abstract parent element-component-directive.

Sections use the mediaStatusChanged event, which is also used to map the
interdependence of media elements

This allows showing the contained text in multiple columns.

There were several issues with it's properties, which could not be 
changed.
Now it is properly initialized and prop change get actually handled.

gridColumnStart to gridColumn, for example.