Remove filtered items from the model if their parent folder is collapsed
Review Request #109343 - Created March 7, 2013 and submitted
Very similar to https://git.reviewboard.kde.org/r/108976/. Basically, the same problem that happens when deleting expanded folders with hidden children is also there when collapsing the expanded folder. The fix is the same. I mostly copied the code from slotItemsDeleted() to setExpanded(), rather than factoring it out, to make merging into master easier.
Works for me, old and new unit tests pass.