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/dynamicelementtextitem.cpp

@@ -23,6 +23,7 @@
 #include "../qetgraphicsitem/conductor.h"
 #include "../qetgraphicsitem/terminal.h"
 #include "../qetinformation.h"
+#include "../utils/qetutils.h"
 #include "crossrefitem.h"
 #include "element.h"
 #include "elementtextitemgroup.h"
@@ -96,7 +97,7 @@ QDomElement DynamicElementTextItem::toXml(QDomDocument &dom_doc) const
 	root_element.setAttribute("uuid", m_uuid.toString());
 	root_element.setAttribute("frame", m_frame? "true" : "false");
 	root_element.setAttribute("text_width", QString::number(m_text_width));
-	root_element.setAttribute("font", font().toString());
+	root_element.setAttribute("font", QETUtils::fontToString(font()));
 	root_element.setAttribute("keep_visual_rotation", m_keep_visual_rotation ? "true" : "false");
 	
 	QMetaEnum me = textFromMetaEnum();