first try using NSCollectionView
question list displays all questions of a presentation
question list only lists questions of the current event
renamed QuestionListGroupHeaderView to QuestionListSectionHeaderView
QuestionListItems: click -> go to Slide, double click -> edit question
clicking on QuestionListItem shows the slide in Powerpoint/Keynote
QuestionList cleanup
update question list if slideIdentifierToSlideNumberMap changes
Merge branch 'collectionView'
nextSlideIdentifier calculated correctly and Keynote slideID tags are not removed if moving a questions not linked to a slide to another slide