Added a first attempt for XML embedding in the main core XML.

Added a first attempt for XML embedding in the main core XML.

Original commit message from CVS:
Added a first attempt for XML embedding in the main core XML.
Objects will emit a signal that a user app can connect to in order to
insert its XML into the tree.
You can catch the object_loaded signal in GstXML to parse the user
supplied XML data in the stream. The object_loaded signal is implemented
with a custom made class signal. All GstObject classes now automatically
create a GstSignalObject that serves as a proxy to the user app when an
object is loaded. All objects are currently responsible to emit the
class signal themselves.
runxml and createxml serve as an example how the XML hooks can be used to
insert and retrieve custom XML tags.