printers: Provide a scheme to address parser

This merge request adds a scheme to the address which we test for correctness by g_network_address_parse_uri() in the new printer dialog. It does not work without it. It uses "none" scheme if user did not entered one.

It also changes default port number to 0 (it is not actually used for connection).

I'll get this also to 3.34 when accepted.

This fixes #679 (closed).