Properly display parent folders when filtering is active and items are added or refreshed

The methods slotRefreshItems(), slotItemsAdded() and slotItemsDeleted()
are adapted so they are now compatible with the new behavior of
filtering expanded folders achieved in
ed83f37f. The new behavior is that
expanded folders are hidden from view if neither the folder itself
nor any of its contents passes the filter. Previously expanded folders
would always stay visible no matter the filter.

A bug where the parent expanded folder would disappear after adding
or refreshing an item while filtering is also fixed.

Tests are added to make sure these behavior changes won't regress.

BUG: 442275
FIXED-IN: 22.04