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/properties/terminaldata.cpp

@@ -17,6 +17,8 @@
 */
 #include "terminaldata.h"
 
+#include "../utils/qetutils.h"
+
 #include <QGraphicsObject>
 
 #include <QDebug>
@@ -115,7 +117,7 @@ QDomElement TerminalData::toXml(QDomDocument &xml_document) const
 		xml_element.setAttribute("show_name", "true");
 		xml_element.setAttribute("label_x", QString::number(m_label_pos.x()));
 		xml_element.setAttribute("label_y", QString::number(m_label_pos.y()));
-		xml_element.setAttribute("label_font", m_label_font.toString());
+		xml_element.setAttribute("label_font", QETUtils::fontToString(m_label_font));
 		xml_element.setAttribute("label_rotation", QString::number(m_label_rotation));
 		xml_element.setAttribute("label_halign", static_cast<int>(m_label_halignment));
 		xml_element.setAttribute("label_valign", static_cast<int>(m_label_valignment));