Lists with this setting delete dropped value objects. This allows 
putting items back.
(Only makes sense when used with copyOnDrop. Otherwise there can not be 
duplicate value IDs).

Also improve constructor value checks to correctly handle nullish 
booleans.

#281

- This allows to reset Geogebra to the state defined in the unit
definition

- Convert player release notes to markdown file

This was necessary because the new text selection action of the button
is not a navigation.

- Areas are marked with the surrounding 'aspect-anchor' tag

The extension is needed to define text areas in the rich text editor
and make them available for the button component

This way images can't be drag-and-dropped into droplist elements.
Also avoids confusion about what is draggable on a page.

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.

- Reduce redundant code
- Prepare implementation of input limitations

The updateListener does not detect all changes inside a GeoGebra element

#309

The API has been updated and some commands and fields are removed. They 
are kept in for compatibility.

Re-implement DropList element

Reads file as base64.

Also rework the GeoGebra dialog to close automatically when a valid 
definition is available, either through pasting or file upload.

- Remove unused variable
- Add typing

- Part of the properties panel for every element (like delete and 
duplicate)
- This allows interacting with elements, generating events and inpect 
their usage. Usually this is unwanted since it might interfere with 
dragging the overlay, but sometimes you want to get a feel for the 
element or check it's contents (GeoGebra for example)

Usually all errors should be caught by the sanitizer, but if that fails 
the editor will be in an okay state.

Also set geometry element to always use fixed size, which is all that 
works anyway.

...but disable the list auto-generation for bullet and ordered lists.

#226
#369

#370

This way the value of the selection box is displayed initially

- Remove property dropListIDs
- Remove mousedown event