Separate autodetected from locally configured printers in QPrintDialog

With QPrintDialog in X11, there is no way to tell the difference between a local printer and a remote printer automatically detected by cups.

The main reason why Debian/Ubuntu does not enable automatic printer detection
in cups by default is that there is no way to tell apart locally configured
(and thus trusted) printers from those which got detected automatically (and
thus open the possibility of luring someone into printing confidential material
onto a rogue network printer).

It would be great to separate the locally configured from
the remotely detected ones:

They can be told apart with the CUPS_PRINTER_DISCOVERED flag (see cups/cups.h)