XML_NAMESPACE_LO_EXT can be used for elements and attributes that are
not yet specified by OpenDocument.

Change-Id: Id29392533d46f6592d964ce79c05ffefa4d69ebc
Reviewed-on: https://gerrit.libreoffice.org/5419Tested-by: Thorsten Behrens <tbehrens@suse.com>
Reviewed-by: Thorsten Behrens <tbehrens@suse.com>