Remove the GList holding printing devices and use already present
GtkListStore for this in the New printer dialog.
This avoids us to install wrong printer when there is more devices
with the same name since we have GtkTreeIter of the selected row.
Use GtkTreeModelFilter for filtering of current devices.
Update test for canonicalize_device_name() since I had to change one
of its parameters.
https://bugzilla.gnome.org/show_bug.cgi?id=749830
'device-class' property of PpPrintDevice can contain "network"
or "direct" values. This information can be stored in already
present property 'is-network-device' as well.
https://bugzilla.gnome.org/show_bug.cgi?id=749830