Module ProspectLists not in Studio? Which *defs.php files aren't recognized in Custom folder?

I was trying to remove the assigned user column from the ProspectLists pop-up.  Seems moving popupdefs.php into custom folder doesn't work. 

I noticed there is a blank studio.php in subpanels but Target List module doesn't appear in Studio.  Can ProspectLists module not be added to studio?