Add WS-Discovery support

The MR adds WS-Discovery support (ie. the wsdd backend and its integration to the network backend), which uses the https://github.com/christgau/wsdd project and requires wsdd binary to be available in $PATH. It also changes the way how files from the smb browse backend are shown by default (ie. the Windows Network folder is replaced by the list of concrete services instead). This MR also contains various fixes and improvements related to the network, smbbrowse, and dns-sd backends. See commit descriptions for more details.

Fixes: #506 (closed)
Fixes: #679 (closed)
Related: nautilus#2785
Related: #307 (closed)