When used, we instantiate the XmlImageStyle class but all its
methods are static, so instantioation is completely pointless.
So change XmlImageStyle to a namespace and static method to
functions.

Change-Id: I6385bd7eeb08c627cb8e48b79a6820372cf94a65
Reviewed-on: https://gerrit.libreoffice.org/47851Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>