core: Add icon to preferences UI's desktop file
core: Add icon to preferences UI's desktop file

Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Tarnyko/rygel,GNOME/rygel,GNOME/rygel,luisbg/rygel,Tarnyko/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel