- remove 'none'

Also restructure files containing interfaces.

- 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

DragAndDropValueObjects are now a sub-interface of TextImageLabel, which 
requires a set imgSrc and imgPosition property. (The reason is that the 
editor needs a way of generating Labels therefore needs the imgSrc set 
to something else but undefined.)

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Rename some test-files

elementModels are described in the unitDefinition
elementCodes are saved in the unitState

Their values are mapped by the ElementModelElementCodeMappingService