New CPDB print backend for GTK Print Dialog

New CPDB print backend for GTK Print Dialog

The Common Print Dialog Backends (CPDB) concept has GUI-toolkit-independent
backends for each print technology (CUPS, Print to File, cloud printing
services, ...) and each print dialog (GTK, Qt, Chromium, ...) is supposed
to use this backend, so that changes in print technologies can be centrally
and quickly covered by changing the backends and everything new gets available
in all print dialogs.

This commit provides a GTK print dialog backend to add support for the CPDB
concept. It communicates with all installed CPDB backends and so gives support
for all these print technologies to the GTK print dialog.

To make use of CPDB the GTK print dialog is supposed to be installed with this
backend and the 'Print To File' backend, and not any others to prevent printer
duplication.