`tabulated-list-set-col' adds a help-echo property to the "labels" that
it inserts.  With this patch this is done a bit differently:

* If the label already has a help-echo property then use that help-echo
  instead of overriding it with a generic one.  This allows displaying
  additional information when it makes sense.
* Otherwise propertize as before but don't prefix the help-echo with the
  column name.  That information is not really useful.