Fixed #682 Error if a group was added to a list of speakers
Merge pull request #691 from ostcar/issue-682

Fixed #682 Error if a group was added to a list of speakers
Update Chapter Agenda in German documentation. Rename various doc files.
Update German documentation Einführung, ErsteSchritte, Tutorial and some others. Screenshots are still missing. Add placeholer icons.
Update German documentation - Agenda
Rename files in German documentation, move images, some small changes.
Update German documentation: Motion