Read-only math fields are displayed without borders

- Show the error message only after the math field has been touched
- Add styles for error message

- Mainly highlight border and moving elements.
- Centerpiece is: line-height: 130%;
  Don't know why or how but it works okay.

- Extra span element around cloze child elements served no visible 
purpose. It also makes sense that an inline div is not inside of a span, 
which is meant for text.
- CSS rules were overengineered and in part shadowed (and thereby 
pointless) by element styles from the template.
This is also in preparation for the DropList styling overhaul.

- Add own line height style to error message to exclude it
- Add missing semicolons to styles

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

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

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

Otherwise, no elements can be dropped at this place

To detect if a custom form element was touched it is necessary to set a
custom tab index

Use the source list placeholderIndex instead of the index for the target 
list.

Chromium does not allow DOM manipulation on dragstart. Therefore add 
timeout.

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

- Using the Geogebra method 'getBase64' is very resource intensive.

This is necessary to avoid playing parts of audios and videos twice
when a unit is reloaded, since audios and videos send their playback
time only every 5 seconds when they are played back.

- Make sure that the textarea has been displayed once (AfterViewInit)
  before its dimensions change again

- The width of the border is no longer needed because
"box-sizing: border" is used
- Scaling of the inner triangle areas is replaced by the setting of 500%
 width and height to be able to fill the whole area of the obtuse
 triangles and at the same time not change the width of their borders