9333979980

Adapt XML exporter logic to the simpler XML importer logic (#657)

The XML importer uses `void System::FillXML(Cell*, wxXMLNode*, bool)` that does not make use of
inheritance by comparing with the parent cell, while the XML exporter in
`wxString Cell::ToText(int, const Selection&, int, Document*, bool)` makes use of inheritance
to avoid emitting repeating attribute values.

This commit adapts the XML exporter to the simpler logic of the XML importer.