gst/gsttaglist.*: Do as tim pointed out and actually register the new tag. Also improve te docs and use gst_tag_merge...

Original commit message from CVS:
* gst/gsttaglist.c:
* gst/gsttaglist.h:
Do as tim pointed out and actually register the new tag. Also improve
te docs and use gst_tag_merge_strings_with_comma() method to allow
retriving all keywords merged in one list.