more subdued colors for the preference buttons

maybe find a better way of doing that?
added intro_text and todo_text to the FilteredListViews and some of the templates
correct verbose name
standard tff template and FilteredView can deal with additional preliminary text
FilteredListView no longer trips if no filter present
List Leistung now works based on FilteredListView
todo text added
simpler buttons for the listings forms
bug fix: tff template only shows filter if there actually is any
final understood how radio buttons really work. Fixes weird bugs with multiple times selecting different buttons