#608

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.

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

The unit tests errored out because of circular dependencies via the 
Elementfactory class. Therefore:
- No longer creates styling/position/player-Properties. This is done in 
the Element parent class.
- Move element creation from Section to the ElementFactory class 
instead.

- Make position props on buttons optional
- Move UIElement creation back to Factory instead of within Section

- 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

There should never be the case, therefore error out.

- Use Unit's getAllElements method of Unit instead of
UnitUtils.findUIElements to get all DragNDropValueObjects
in UnitComponent
-Remove unnecessary timeout in UnitComponent

Move label creation to ElementFactory.

No way has been found to have an abstract static method for 
OptionElement's label generation, therefore silence the linter.
Same for getElementComponent.

- Missing type inforamtion as constructor parameter
- Also the new ID generation mechanism uses the type of the element to 
request an ID from the the IDManager. Here was a mismatch, where the 
type is 'likert-row', but the IDManager only knew 'likert_row'. This is 
now files all IDs use hyphens instead of underscores.

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

- 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.

- Now only UIElements are found
- HTML parts of the text editor are ignored

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.

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.

With this setting elements are copied when being dropped to another 
list.

On drop it is also checked if the item-ID is already present in the 
list. If it is the drop event is silently discarded. This allows putting 
items back in the list without creating duplicate IDs.

Lists with this setting:
- do show a placeholder of the items being dragged. This way it is 
conveyed that the item will remain there after being dropped.
- don't show a placeholder when items are hovered over them, to avoid 
confusion with duplicate items.

Sorting within DropLists is disabled, since it causes moving items 
within the list. It has to be discussed if this feature is even needed.

- 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.

#243

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

The package karma-spec-reporter is to show more info about run tests on 
the console.

Numbers on the test files refers to the version (2 digits for minor; 
patch number ignored).

New name: richTextOptions

Unit definition version has been raised accordingly.

Use own method `handleRadioButtonGroupElement`

This reverts commit d053d03c.

This feature will first be developed properly in a branch.