#513

- Can also be used with other elements

In Player-context the constructors are to fail, when the given 
properties are not exactly as expected. Therefoore they are validated. 
When this validation fails, an environment variable is checked to 
determine if the element should be created anyway with defaults. This 
switch is off in player-context. But the Editor is not so strict and can 
create elements with incomplete parameters.

There is also a new VersionManager which checks given unit definitions 
for version-validity. Upon failing this check Player and Editor show 
error messages.
The Editor accepts a certain unit definition version, but shows a 
warning.

The Editor checks units for invalid references after loading. If any are 
found, they are removed and an info-panel is shown.

- Remove dynamicPositioning from PositionProperties. This is not 
knowledge that the element needs to know. In the few places it was used, 
it was fixed or solved by asking the containing section about which 
positioning is active.

- Fix and improve cloze child sizing. All can now have dynamic width and 
height.

- Fix empty lines in ClozeDocument rendering

- Editor SelectionService now knows if a cloze child is selected. This 
way the PropertiesPanel can show valid dimension inputs regardless of 
the underlying section.

Measure holds the value and a unit for CSS properties.

- Replace UpdateTextareaPipe with DynamicRowsDirective

The dynamic height of the textarea is calculated based on the available
width. However, accessing the width of the textarea often results in an
error.
For this reason, the calculation is now done using resize and change
events in conjunction with a timeout to wait for the textarea to render.

- Prepare usage in editor
- Remove test from ScrollPagePipe, because the call to "new Page" expected
  the initialization of GeometryComponent

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

- Add pipe to decide which list to use

Uses external software 'MathLive'.

Distpacker issues with mathlive have been solved in a custom 
distpacker.ts in scripts-folder. This is now used instead of the 
distpacker from the npm package.

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

- 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

Create new pipe for checking if element is included in list.

Using the html property "disabled" results in a warning message in the
player when reactive forms are used at the same time.

ViewEncapsulation.None is required for overriding tooltip

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.

- Split text field and drop list, so they can have differing props
- Improve their dimension default values
- Improve alignment of all cloze children components
- Add dynamic width parameter of toggle buttons to dimension properties
panel. Therefore the postion tab can always be shown and no longer needs
a condition.

Instead, use Textfied component with the property isClozeChild

This helps to divide the code of the element container into smaller
meaningful units.
In addition, each element can be given its own form group.

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.

Compound elements are now supposed to use overlays for their child
elements. This overlay makes selecting child components (by clicking on
them) and marking them as selected (done by the selection service)
possible.
The SelectionService no longer needs special logic to handle compound
children selection, as they now also have an overlay with the same
interface as normal (canvas) elements.

A few modifications in connected directives are necessary. The now
handle children components in a proper array instead of QueryList.

Likert elements do not have clickable children yet and work a little
differently. This should probably be unitized in the future.

- Implement marking buttons as toggle buttons

Is now able to interpret more stuff from the editor. The only thing 
missing are the list styles.