More robust row selection handling.

For yet unknown reasons (buggy proxy model stack possibly) we sometimes
end up with selections that don't span all columns (or even change their
column span over time). As a result the action state is not properly
updated, leaving you with disabled actions for the corresponding
items/collections.

This definitely fixes the non-working folder properties action in
akonadiconsole, it might fix similar effects in KMail that are only
observable occasionally.

Reviewed by Stephen Kelly.