Most importantly dynamic sections are now default.

Cloze documents now use this correctly.

#233

- 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.

verticalButtonAlignment is the new property for likert rows, which tells 
the actual radio button to either be at the top or in the middle.

- A new new editor component has been created for this, which has a
reduced interface and produces span-elements instead of paragraphs.
- Since the created HTML (span with CSS) is regarded as potentially
harmful it needs to be sanitized. For element components the sanitizer
is put in the abstract parent element-component-directive.

Sections use the mediaStatusChanged event, which is also used to map the
interdependence of media elements

This allows showing the contained text in multiple columns.

There were several issues with it's properties, which could not be 
changed.
Now it is properly initialized and prop change get actually handled.

gridColumnStart to gridColumn, for example.

This can be enabled for TextFields and TextAreas and shows a keyboard
at the bottom of the screen. This does NOT set the field to read-only
which is probably wanted in a situation where the system keyboard is not
to be used. For flexibility purposes it is not implicitly set and needs
to be set manually. The same goes for other input assistance settings,
which should not be used in addition to the keyboard but it is allowed.

The keyboard has an extra switch to additionally show french special
characters.

This makes newly created elements appear next to each other instead of 
on top.

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

becomes

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

Instead, use Textfied component with the property isClozeChild

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

The factory was ignoring the src parameter, which is set before by the 
UnitService.