- 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!)

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.