HTML labels can cause errors in the Test Center for units in XML format.

Development

[player] Fix deletion of text when removing markers on the text element

Feature/mathfield improvements

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

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

- 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

This reverts commit f90bcc5b
Target installation seems not to support this Verona Api version yet

Mathlive package requires some more space.

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