sort task-lists for estimated / completion / #974

This would be a really nice feature, but the current list-handler does not cover this feature. This might be possible by adding an array as column-header:

• "Estimated / Complete" would be defined as

title= "%s / %s", 
tooltip= __("Some tooltip"),
sort_keys= array(
    array(
    field='estimated',
    name=__('Estimated'),
   ),
   array(
    field='completed',
    name=__('Completed'),
  ),
)