QV: Filters can not be exported/imported

Two changes that a) add an event so that imported filters are added to the combo box in the UI and b) referenced axes in the XML (referenced by name) which have been deleted are replaced with an arbitrary default axis when loaded from the defaults. (Users should not delete an axis that is used in a chart/table etc, but currently there is no logic that keeps them from doing that, and I somewhat think it's actually not worth implementing that.)