BUG-1382: deprecate QName.getPrefix()

Prefix storage in QNames is deprecated. Mark the constructors and
getter as deprecated so it can be removed at a later stage.

Change-Id: I68637fe156195b3668cce334b1cc70ab322c0ffc
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1382: deprecate QName.getPrefix()"