Corrected logic for adding 'fav' class to a just favourited question
[qzb-filters]: Re-implemented how filters work during quiz building ...

The new scheme relies on class attributes being set on .single-lines
and a [filter] attribute set somewhere up the hierarchy

Currently there are 3 filters - for favourites, selected and show-all.

KPNS: Basis for showing selected questions is wrong
[qzb-filter]: Removed old code