- Add 'Button' as possible child of cloze elements

The property vertical-align is interpreted differently by Chrome based
browsers, so a special webkit value is needed to override the value. The
browser specific value is used last, so it is used by browsers which can
interpret it.
#414

- text field simple
- toggle button

#405

#381

#409

- Add left and right padding for input field

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

- When the height of the child elements of a cloze element is increased,
they all now grow out of their center

Read-only math fields are displayed without borders

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

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

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

- Convert player release notes to markdown file

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

This version is unreleased and can be used.