Add support for CUPS printer instances

Each CUPS printer instance is handled as a separate
GtkPrinter that initially has the same values as the
printer's default instance (except for the name).

The specific values for the instances are retrieved
when the user values for the printers are evaluated.

https://bugzilla.gnome.org/show_bug.cgi?id=148184
2 jobs for michaelweghorn/supportCupsPrinterInstances in 8 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #100014
build

00:08:07

manual #100015
win32 allowed to fail manual
msys2-mingw32