SAXEventKeeperImpl is the only implementation of the
css::xml::crypto::sax::XSecuritySAXEventKeeper interface, and this way
it's not necessary to cast to various interfaces explicitly at a lot of
places.

Change-Id: I25f76c88d5fefef42a6e8394a9d6de98c52b12c0
Reviewed-on: https://gerrit.libreoffice.org/35317Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>