Implement STRUCTURE_CHANGED messages. These messages will be used to signal the parent bin of link/unlink operations ...

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstmessage.c: (gst_message_new_structure_change),
(gst_message_parse_structure_change):
* gst/gstmessage.h:
Implement STRUCTURE_CHANGED messages. These messages will be used to
signal the parent bin of link/unlink operations that could require a
resync when doing a state change. See ##510354.
API: gst_message_new_structure_change()
API: gst_message_parse_structure_change()