Fixes: #152864

Fixes: #152864
Original commit message from CVS:
* docs/libs/gst-plugins-base-libs-sections.txt:
* ext/alsa/gstalsamixer.c:
* ext/alsa/gstalsamixer.h:
* ext/alsa/gstalsamixerelement.c:
* ext/alsa/gstalsamixertrack.c:
* gst-libs/gst/interfaces/mixer.c:
* gst-libs/gst/interfaces/mixer.h:
* gst-libs/gst/interfaces/mixeroptions.c:
* gst-libs/gst/interfaces/mixeroptions.h:
* gst-libs/gst/interfaces/mixertrack.c:
* gst-libs/gst/interfaces/mixertrack.h:
* tests/check/Makefile.am:
* tests/check/libs/mixer.c:
Patch By: Marc-Andre Lureau <marcandre.lureau@gmail.com>
Fixes: #152864
Add support for notifying mixer changes on the message bus, and
implement it in alsamixer.
API: gst_mixer_get_mixer_flags
API: gst_mixer_message_parse_mute_toggled
API: gst_mixer_message_parse_record_toggled
API: gst_mixer_message_parse_volume_changed
API: gst_mixer_message_parse_option_changed
API: GstMixerMessageType
API: GstMixerFlags