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.

This makes for smaller more managable components.

to ensure that a section that is not displayed has no dimensions

Call emitter in template and remove unnecessary method

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.

The material dialog functionality used, adding the mat-dialog-title 
directive the toolbar.

The text area was mistakenly shown.

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

This helps identifying the selected element.

gridColumnStart to gridColumn, for example.

The number row has alternative keys, activatable via shift.

Also add new keys for dot, comma etc.

For some reason the 'host' selector does not work properly. Switching 
back to using the component selector.

This can be enabled for TextFields and TextAreas and shows a keyboard
at the bottom of the screen. This does NOT set the field to read-only
which is probably wanted in a situation where the system keyboard is not
to be used. For flexibility purposes it is not implicitly set and needs
to be set manually. The same goes for other input assistance settings,
which should not be used in addition to the keyboard but it is allowed.

The keyboard has an extra switch to additionally show french special
characters.

Don't use the page maxWidth when it is not set.

This makes newly created elements appear next to each other instead of 
on top.

It is enough to set the row and column values. Spans are now only 
changed from the props panel.

This had to many implementation problems and is not really an important 
feature. Removing the corresponding stylings also improves the visual 
layout - elements can now grow freely and the section resized with them.

This is now done only via the cdkDropListGroup directive which collects 
child drop lists. There is actually no need to connect them manually.