Don't hide address entry when there is not firewalld available.
It is still possible that firewall is set up correctly and user
should be able to enter address of his network printer and
install it (rhbz #743694).
This commit enables services mdns, ipp, ipp-client and samba-client on
firewall for 5 minutes for detection of network printers (#648784).
It enables required services permanently for printers selected by user
for addition then. It shows a notification for the permanent enable.
It uses firewalld, so if it is not installed or running it shows a warning
message to the user in the place where discovered printers will be shown
(both local and network). The warning disappears after the finish of
printers discovery (but not from network part if no printer was found).
This patch makes PpNewPrinterDialog to call DevicesGet method
asynchronously now.
It also adds spinner to the lower left corner with label showing
status of this operation (#644212).
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).
