NewDiagramPage::applyConf() writes every other default to QSettings —
border, title block, conductors, folio reports and the guides — but the
cross-reference branch only fetched the properties into a local hash and
then dropped it on the floor. hash_xrp was never used.
The result: changing the cross-reference defaults under Settings > New
project has no effect. Nothing is written, no defaultxref* key ever
appears in the configuration file, and XRefProperties::defaultProperties()
keeps handing out the hardcoded fallbacks for every new project.
Write each of the four types (coil, protection, commutator, plc) with the
"diagrameditor/defaultxref" + key prefix that defaultProperties() already
reads back.