Get the name of the first printer in a class to use when requesting a PPD

Get the name of the first printer in a class to use when requesting a PPD

* modules/printbackends/cups/gtkprintbackendcups.c:
  (cups_request_printer_list_cb): Get the name of the first printer
  in a class to use when requesting a PPD
  (cups_request_ppd_cb, cups_request_ppd): Fix the parameters of
  the "details-acquired" signal emittion
  (cups_request_ppd): get the ppd name from the cups printer

* modules/printbackends/cups/gtkprintercups.c (gtk_printer_cups_get_ppd_name):
  New method for getting the name of the ppd we should request