mixer-control: Let source update handle active output change

mixer-control: Let source update handle active output change

A signal is emitted when the output changes so don't set it right away
as the stream might change port.

This also brings set_default_source in line with set_default_sink()

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Origin: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/merge_requests/31
Bug: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/issues/34
Bug-Debian: https://bugs.debian.org/1126514
Applied-upstream: gnome-shell 50, commit:798230599950607ec285daecd070f5bcb28580e4

Gbp-Pq: Topic libgvc
Gbp-Pq: Name mixer-control-Let-source-update-handle-active-output-chan.patch