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
4 jobs for michaelweghorn/supportCupsPrinterInstances in 14 minutes and 40 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #302277
fedora-autotools

00:10:14

passed #302278
fedora-meson

00:07:45

manual #302280
win32 allowed to fail manual
msys2-mingw32-autotools
passed #302279
win32
msys2-mingw32-meson

00:14:39