Added GstStructure to gst_value_table and its related functions.

Added GstStructure to gst_value_table and its related functions.

Original commit message from CVS:

* gst/gstcaps.c: (gst_caps_to_string),
(gst_caps_from_string_inplace):
* gst/gststructure.c: (gst_structure_get_abbrs),
(gst_structure_to_string), (gst_structure_from_string):
* gst/gstvalue.c: (gst_value_set_structure),
(gst_value_get_structure), (gst_value_serialize_structure),
(gst_value_deserialize_structure), (_gst_value_initialize):
* gst/gstvalue.h:
* tests/check/gst/gststructure.c: (GST_START_TEST),
(gst_structure_suite):
* tests/check/gst/gstvalue.c: (GST_START_TEST):
Added GstStructure to gst_value_table and its related functions.
Changed gst_structure_to_string to print ';' in the end.
Changed gst_caps_to_string to not print ';' beteween its
fields (structures) anymore and remove the lastes ';' from latest
structure. Now it is possible to have nested structures.
In addition, backward compatibilty is assured by accepting '\0' as
end delimiter. Fixes: #487969.
API: add gst_value_set_structure()
API: add gst_value_get_structure()