send bulk mail from search results: should not send and continue

if a bulk email is generated from search results, we do not allow the user to save and return at a later date to continue the mailing.
if a search is conducted and "all" records are selected, the civimail wizard (from the action dropdown) correctly hides the save and continue button.
but if a search is conducted and only a few records are selected, the button is hidden on step one, and visible on all subsequent steps.

this is because our condition is: $this->_searchBasedMailing && $this->get( 'ssID' )
(or a variant) – and ssID only exists if we have a saved search (i.e. "all" records were selected)

the condition should only be: $this->_searchBasedMailing

that said – I really would like to see us revisit this limitation. it would be great if we could save the mailing recipients for search based mailings. the facility to do that is there (since they are all just stored in a hidden group). it's more a matter of communicating accurately what we are doing (as users may be confused as to whether their original search is re-run when they continue the mailing at a different time)