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

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

Just called from the editor, before creating the Unit instance.
It goes through all IDs and checks for duplicates. If found it generates 
a new one and shows the prompt.

Transform string array to Textlabel array.

#338

had some issues with the switch to the new options stuff.

When calling the element constructor may generate IDs. Those appear as 
already registed later when the unit is instantiated for real.

- Sanitize without using unit definition version

- 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!)

The equality check compares the reference of the array and not the
values of the array.

sanatizer
TextImageLabel:
 position -> imgPosition

DragNDropValueObject
 stringValue -> text
 imgSrcValue -> imgSrc

RadioButtonGroupComplexElement
 columns -> options

Likert
 columns -> options

#298

Remove 'any's be adding proper types and using 'never'.

Now properly finds all normal and simple drop lists.
This is achieved by a new method of unit all it's sub-components, which 
returns all contained elements with and optional type filter.

Also improve (still not very pretty) styling of the drop list props 
panel.

#260 #259

The service is no longer an Angular service, but is now a basic 
singleton.

The creation of a temporary cloze elements fails when it encounters 
unknown child types.
Therefore we use a static ClozeElement method to get the child elements 
from the document instead of the whole element.

- Elements initialize their fields explicitly and not with
  Object.assign, since Object.assign also assigns
  (and therefore overwrites) parent fields.
- IDService is no longer an Angular service but a basic singleton. This
  allows to use it in classes.
- The IDManager can be optionally passed to unit and element
  constructors. When present the elements check and fix their given ID
  when created.
  This way elements can be created without depending on the external
  IDManager but also with the dependency.
- ElementFactory is no longer used to create elements. Those are now
  either created directly via 'new' or via the Section, which knows the
  different element types.

This is to convey a successful operation.

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.

- Expand the list of elements zo sanitize to
['text-field', 'text-area', 'text-field-simple', 'spell-correct']
- Rename sanitize method
- Change expectations in tests

'text-field' and 'drop-list' now need '-simple' suffix.

Makes more sense logically to pass the cloze element as a whole instead 
of just the document. Knowing about the document is the responsibility 
of the method not the caller.