hwmixvolume: replace PyGTK with gobject-introspection

hwmixvolume: replace PyGTK with gobject-introspection

This doesn’t work yet, we require Gtk 3.0 rather than 2.0 and the API
changed quite a lot, so this is but a preparatory patch.

This is done so that we can get rid of GTK+ 2 which has been EOL for
many years already, and to add Python 3 support because Python 2 will
very soon be EOL as well.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>