[BUGFIX] Fixed hasTeaser check

The hasTeaser check counted the default content elements instead of
the teaser content elements to check if a teaser is present. This
is now fixed.
[TASK] Replaced spaces with tabs in TypoScript setup
[TASK] Added / updated some PhpDoc comments
[TASK] Method for adding teaser contents

The NewsRichteaser domain model has now a method for adding content
elements.