Measure holds the value and a unit for CSS properties.

Also unit defintion version to 3.10.0

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.

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

- Add the new features from version 3.7.0
- Fix the entries from version 3.8.0

- add hotspot image element
- raise version to 3.8.0

- Add "isOlderThan" method to the "SanitizationService" to distinguish
old ToggleButtons with the property "option" from new ones
- Use SanitizationService.isUnitDefinitionOutdated in Player to set
the unitDefinitionVersion (should be modified!)

New name: richTextOptions

Unit definition version has been raised accordingly.

verticalButtonAlignment is the new property for likert rows, which tells 
the actual radio button to either be at the top or in the middle.