Deleting/Disabling a group removes its mailings from CiviMail

[dalin]

This is somewhat related to CRM-1794

Currently the list of mailings at /civicrm/mailing is generated by first searching for all enabled mailing groups, then looking up what mailings are sent to these groups.  Thus if a group is disabled or deleted its mailings no longer show in the list. 

This is extremely confusing for CiviMail admins to have their mailings all of a sudden disappear. 

We need to come up with a good plan of if and how deletes are cascaded through from groups right down to click tracking and activity records.  Then have a good solid UI to make the process as usable as possible. 

IMHO a group should not be allowed to be deleted if it has associated mailings.