- 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.

The sanitizer is now a service, so it is testable.
Since it uses the IDService it needs to be a real service as well.

Both are used by the player therefore moved to common.

- Clearer methods with only one return value
- better naming
- comments

New name: richTextOptions

Unit definition version has been raised accordingly.

Use own method `handleRadioButtonGroupElement`

- Fixed dynamicPositioning property of all section children. This 
property was accidentally set with faulty values in the last version.
- Also sanitizes likert and likert row elements, because rows now have a 
TextImageLabel instead of plain text.

gridColumnStart to gridColumn, for example.

gridColumnStart: number;
gridColumnEnd: number;
gridRowStart: number;
gridRowEnd: number;

becomes

gridColumn: number;
gridColumnRange: number;
gridRow: number;
gridRowRange: number;

The unit definition now has 2 meta-fields: 'type' and 'version'.
The former has the fixed value of 'aspect-unit-definition'.
The latters contains the version without any prefixes, i.e. '1.2.3'.

The sanatizer tries to read the versions of older formats as well and
uses the found version. This is important since the handlePlusOne
method must only run of a specific version (in the old format).

The sanatizer no longer shows any messages and therefore does not need
the MessageService anymore. The information about whether sanatization
happened or not is returned as second parameter for those that are
interested. The editor is, the player is not.

The editor API service now constructs the 'unitDefinitionType'
parameter from both values, resulting in something like
'aspect-unit-definition@1.2.3'.

- Units and element are now built via the factory. This ensures only the
  wanted properties are present. All others are discarded.
- Since the sanatizer is supposed to check the unit defintion version
  it is used by player and editor and therefore put in common.
  This in turn makes it necessary to put the JSON resolver settings
  in the root-TSConfig file.
- Renamed a couple of properties to have clearer names.

Remove manipulations from elementModel
- Refactor drop list component
- Use of initial values for audio and video playback time

Defining the validators not in the elements anymore, but when creating
the forms

This makes for cleaner imports.

Since the array of 'ClozeElements' can be empty, it must be checked

This removes all the classes and goes back to simple interfaces for
unit, pages etc and all elements. This makes creating them easier and
cleaner.
Things like specific needs when setting element properties and most
importantly compatibility handling is done by a special service module,
that knows all the elements and how to repair old their potentially
outdated definitions.