libjuicer: Add support for the "none" proxy mode

This stops sound-juicer from using a proxy even when that one is
actually deactivated in the preferences.

https://bugzilla.gnome.org/show_bug.cgi?id=729379
libjuicer: Use the glib generic marshaller

Instead of creating a custom one. Note that this is a new function in
GObject 2.30, but we already depend on the associated Gio 2.32, so an
updated version dependency isn't necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=729661