ext/alsa/: Make alsasink/src a subclass of alsamixer so that mixer stuff shows up in gst-rec. Needs some finetuning.

Original commit message from CVS:
* ext/alsa/gstalsamixer.c: (gst_alsa_mixer_build_list):
* ext/alsa/gstalsasink.c: (gst_alsa_sink_get_type),
(gst_alsa_sink_class_init):
* ext/alsa/gstalsasink.h:
* ext/alsa/gstalsasrc.c: (gst_alsa_src_get_type),
(gst_alsa_src_class_init):
* ext/alsa/gstalsasrc.h:
Make alsasink/src a subclass of alsamixer so that mixer stuff
shows up in gst-rec. Needs some finetuning.