Improve accessibility of feed list items.

This is a bit of a hack as we are using AXDescription for this. Overriding AXValue is not viable without
much more code because all dependent parameterized attributes (AXAttributedStringForRange etc.)
would need to be adjusted as well.

Cleaner solution can IMHO be implemented only after transitioning to a view-based outline and have
each element (error image, progress image, count label, feed name, folder image) available
as a subview of the cell’s view. For now, this quite works.