Write font descriptions in the stable Qt 5 10/11-field format

QFont::toString() is not stable across Qt versions: Qt 6.11 switched to
a 19-field format carrying OpenType weights, which QFont::fromString()
of Qt 5.x and Qt <= 6.10 rejects, leaving a broken font. Projects saved
by a Qt 6.11+ build were therefore unreadable by older builds.

Add QETUtils::fontToString() composing the legacy 10/11-field
description (weight mapped back to the legacy scale with the same
closest-match table Qt uses when parsing) and use it at every site that
stores a font description in a project, element, table config or
settings file. Every Qt version from 5.15 through 6.12-beta parses this
form correctly, so files stay readable by every QET build in
circulation.

See issue #553.

sources/qetgraphicsitem/independenttextitem.cpp

@@ -21,6 +21,7 @@
 #include "../diagramcommands.h"
 #include "../qet.h"
 #include "../qetapp.h"
+#include "../utils/qetutils.h"
 
 #include <QDomElement>
 #include <QSettings>
@@ -80,7 +81,7 @@ QDomElement IndependentTextItem::toXml(QDomDocument &document) const
 	result.setAttribute("y", QString("%1").arg(pos().y()));
 	result.setAttribute("text", toHtml());
 	result.setAttribute("rotation", QString::number(QET::correctAngle(rotation())));
-	result.setAttribute("font", font().toString());
+	result.setAttribute("font", QETUtils::fontToString(font()));
 	
 	return(result);
 }