- Replace foreach loops with map methods

- to prepare markable support for text as compound children

- Input field manipulation moves from service methods to new
  TextInputGroupDirective

#461

- Use own StateVariableStateService which uses the same new base class
as UnitStateService

- Refactor directive

- Basic implementation

- If this property is set, the input of more characters than specified
in maxLength is prevented
- It also prevents the insertion of text.

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 facilitates the testability

Change import paths of CommonModule files in editor and player

Rename KeyboardComponent to KeypadComponent
Rename KeyboardService to KeypadService
Split LayoutComponent into LayoutPlayerComponent and
LayoutPagesComponent
Prepare the introduction of an additional keyboard

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 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outdated definitions.

* Use IntersectionDetector for intersectionDetection directive
* remove unnecessary input 'index' from page component