This commit implements design changes from
https://live.gnome.org/Design/SystemSettings/Printers.
The new printer dialog gets informations about connected devices
from CUPS server asynchronously and separately for each backend now.
Entering an address into the entry and pressing the icon inside
the entry or enter starts to detect printers on the entered host.
Entering a text which is a substring of a name of a device or its location
filters the list to contain just devicess with the string in it (e.g. Canon
will keep devices with "Canon" in their name).
The PpNewPrinterDialog is regular object now. It emits signal "pre-response"
when dialog is closed and a printer is being added and signal "response" when
the new printer was added, addition of the new printer failed or the dialog was
cancelled.
This commit removes FirewallD support from new printer dialog. (#683229)
The dialog is able to add printers discovered by cupsGetDevices().
It can also add printers from a remote CUPS server by typing
its address and selecting printer we want to add (#640734).
