Save the default cross-reference properties to QSettings

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.

sources/ui/configpage/configpages.cpp

@@ -214,7 +214,11 @@ void NewDiagramPage::applyConf()
 		rpw->toSettings(settings, "diagrameditor/defaultreport");
 
 		// default xref properties
-		QHash <QString, XRefProperties> hash_xrp = xrefpw -> properties();
+		const QHash<QString, XRefProperties> hash_xrp = xrefpw->properties();
+		for (auto it = hash_xrp.constBegin() ; it != hash_xrp.constEnd() ; ++it) {
+			it.value().toSettings(settings,
+						  QStringLiteral("diagrameditor/defaultxref") % it.key());
+		}
 
 		// Global in QSettings speichern
 		QList<Diagram::Guide> current_guides = m_gpw->guides();