gst/: Use parent refcount in GstMessage to ensure GstStructure consistency.

gst/: Use parent refcount in GstMessage to ensure GstStructure consistency.

Original commit message from CVS:
* gst/gstmessage.c: (_gst_message_copy), (_gst_message_free),
(gst_message_new), (gst_message_new_error),
(gst_message_new_warning), (gst_message_new_tag),
(gst_message_new_state_changed), (gst_message_new_application),
(gst_message_get_structure):
* gst/gstmessage.h:
* gst/gststructure.c: (gst_structure_set_parent_refcount),
(gst_structure_copy_conditional):
Use parent refcount in GstMessage to ensure GstStructure
consistency.
Cleaned up headers a bit.