Make the EntityRightsFilterModel private.

The proxy attempts to to recursive filtering, but does not keep
up-to-date with the source model. Rebase it on KRecursiveFilterProxyModel
which does, but which will not be public API until 4.5.

CCMAIL: kde-pim@kde.org

svn path=/trunk/KDE/kdepimlibs/; revision=1056677