Support retention of unrecognized styles.

I'm currently building some tools that depend on custom style attributes, but Inkscape ignores them when reading and writing.

If you use the XML editing mode to set an attribute on a Layer tag similar to: "-demo: true", and then change the visibility of the layer, this attribute is lost with current main branch build.

This PR changes that; if an attribute is unrecognized, Inkscape will store it in Style::extended_properties.