- Set the duration of highlighting anchors to 15 seconds
- Disable simultaneous highlighting of anchors

Since buttons can now act as switches for text-element highlighting, it 
is no longer exclusively used for navigation purposes.

Rules for when drop is allowed have been fixed. It is quite complicated 
and therefore a lot of explanatory comments are added.

- improved translation
- add example file

Uses external software 'MathLive'.

Distpacker issues with mathlive have been solved in a custom 
distpacker.ts in scripts-folder. This is now used instead of the 
distpacker from the npm package.

Allows DropListvalueObjects which have this setting to be replaced in
lists which only allow onlyOneItem. When this switch is set, the class
sets the other 2 internal values
(originListID?: string; originListIndex) itself. Those can not be seen
nor changed from outside.

Also refactoring in lots of places around droplists. Better names, 
cleaner structure, comments.

#305

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

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

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

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

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

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units