audio/libmatemixer: help mate_mixer_applet to save the "track to control"

• Using mate 1.8.1 installed from packages. Unable to set the volume straight from the slider of the Mixer Applet, until I right click the icon, go to preferences, and select PCM as the track to control. If I restart the mate_mixer_applet (either via kill or regular shutdown) the setting goes back to Volume.
• After the port patch the instelled libmatemixer backend works fine.
• see also: https://forums.freebsd.org/threads/fixing-default-audio-device-in-mate.58957/

PR: 202578
Submitted by: Attila <agm2port@gmail.com>
Reported by: Abilio Marques <abiliojr@gmail.com>
Reviewed by: Walter Schwarzenfeld <w.schwarzenfeld@utanet.at>