Move registry backend API to private headers where we can. Add fixme-0.11 comments for the others. Add stubs for the ...

Original commit message from CVS:
* configure.ac:
* gst/gst_private.h:
* gst/gstconfig.h.in:
* gst/gstregistry.h:
* gst/gstregistrybinary.c:
* win32/common/gstconfig.h:
Move registry backend API to private headers where we can. Add
fixme-0.11 comments for the others. Add stubs for the xml backend when
using the binary to ensure they functions exists (they should not be
used though). Fixes #520756.