#667

As the registration of hidden and visible pages can take place
asynchronously, the array of hidden and visible pages must be sorted

#605

#606

- Prevent input of 'Dead' keys

- A suitable version was not available after update of Angular
- Reactivate test for valid pages

Replace the assignment of the ElementComponent with the assignment of
a ElementComponent that will work in future versions of Angular

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

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.

This eliminates the need to specify the different page types in
AppComponent

to avoid lint errors in templates

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.