- 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

- Rewrite interface 'VopMetaData'
- Add parameter 'metadata' to VopReadyNotification

- 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

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

- This makes the behavior identical to real form elements
- To trigger the focusout event, the HTML element needs
the tabindex attribute

- 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!)