API: add gst_mixer_options_list_changed(), gst_mixer_mixer_changed() and gst_mixer_me...

API: add gst_mixer_options_list_changed(), gst_mixer_mixer_changed() and gst_mixer_message_parse_options_list_changed...

Original commit message from CVS:
* docs/libs/gst-plugins-base-libs-sections.txt:
* gst-libs/gst/interfaces/mixer.c: (gst_mixer_option_changed),
(gst_mixer_options_list_changed), (gst_mixer_mixer_changed),
(gst_mixer_message_get_type),
(gst_mixer_message_parse_option_changed),
(gst_mixer_message_parse_options_list_changed):
* gst-libs/gst/interfaces/mixer.h: (GstMixerType),
(GST_MIXER_MESSAGE_OPTION_CHANGED),
(GST_MIXER_MESSAGE_OPTIONS_LIST_CHANGED),
(GST_MIXER_MESSAGE_MIXER_CHANGED):
API: add gst_mixer_options_list_changed(), gst_mixer_mixer_changed()
and gst_mixer_message_parse_options_list_changed(). Fixes #519916.