This avoids import cycles.

This keeps import cycles from happening.

For now this is only relevant for checkbox values. Would be better to 
have a layer of abstraction of top. Maybe BooleanInputElement.

This makes checking the existence of properties for the property panel
possible, since it is possible to check for null but not for undefined.

to avoid name collision.

There are still some problems. It works a lot better than before. Adding 
this as an intermediate solution.

Before the same array reference was saved for all selected elements. Now 
it correctly creates a copy of the original array before saving it as 
value.

It does not support changing the fnt size.

- Fix conditions for showing inputs. As all the properties are mandatory 
and have to be available, so no contionals are need here.
- Move CSS into the component (from parent properties component)

Importantly this includes moving logic into the classes.

... and no longer an Angular service.
This is because the IDService is needed in classes, where it is not 
injected by default.

- Remove interfaces
- Add classes for all elements as well as unit, page and section
- 2 intermediate interfaces are kept. They dont constituate a viable 
object but only additional properties, like background-color. Those can 
be added as interface to all elements. Initialisation has to be done by 
the element itself though and not by some constructor higher up.

Put the possible element types explicitly.

This was an early draft and unused. Will be reintroduced in a proper 
form later.

The different tabs are now separate components, which makes them a lot 
smaller and better to handle.

This replaces the label text with an image, which can be uploaded.

The existence of props can be easier checked via null-check.

This is rather hard to implement via HTML inline style attributes. So we 
do it the classic way.
Could be reviewed, when that feature is implemented in TipTap:
https://github.com/ueberdosis/tiptap/issues/1514.

Based on TipTap which in turn is based on ProseMirror. No further assets 
are needed.

The editor package is larger than that. So the build fails without the 
higher limit.