printers: Propagate cancellation of authentication dialog

Return "response_id" of PpAuthenticationDialog in its "response" signal.

https://bugzilla.gnome.org/show_bug.cgi?id=698532
printers: Enable use of emblems for printer icons

Use GIcon instead of icon names in PpNewPrinterDialog.

https://bugzilla.gnome.org/show_bug.cgi?id=698532
printers: Authenticate Samba servers only when enabled

Add parameter which enables showing of authentication dialog
for Samba servers which need authentication.
Add samba server to the internal list of found devices if it needs
authentication and the authentication is disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=698532
printers: Handle cancelling of authentication dialog

Add given Samba server to the list of found devices
if authentication dialog was cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=698532
printers: Simplify freeing of lists of found printers

Create a dedicated function for freeing of PpDevicesList.

https://bugzilla.gnome.org/show_bug.cgi?id=698532
printers: Show authenticated Samba servers in the list of new devices

When a Samba server which needs authentication is discovered, add
it to the list of new devices and allow user to authenticate against it
by pressing new "Authenticate" button.
Show only printers available on such a server when authenticated and
remove the server from the list.

https://bugzilla.gnome.org/show_bug.cgi?id=698532