gst/matroska/: Use gst_value_serialize() and gst_value_deserialize() for transforming tags from some GType to a strin...

Original commit message from CVS:
* gst/matroska/matroska-demux.c:
(gst_matroska_demux_parse_metadata_id_simple_tag):
* gst/matroska/matroska-mux.c: (gst_matroska_mux_write_simple_tag),
(gst_matroska_mux_write_data):
Use gst_value_serialize() and gst_value_deserialize() for transforming
tags from some GType to a string and the other way around. The default
transformations in GLib don't include transformations from string to
number types.