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.

- Fix type of selectedGroup

This facilitates the testability

- Reduce amount of properties
- Split code in separate methods
- Define some properties as input properties

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

Knowledge of the 'document' existence is only a matter of the function 
not the caller.