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/ViewItem/projectdbmodel.cpp

@@ -22,6 +22,7 @@
 #include "../../qetinformation.h"
 #include "../../qetproject.h"
 #include "../../qetxml.h"
+#include "../../utils/qetutils.h"
 
 #include <QSqlError>
 #include <QSqlRecord>
@@ -252,7 +253,7 @@ QDomElement ProjectDBModel::toXml(QDomDocument &document) const
 	
 	//Add index 0,0 data
 	auto index_00 = document.createElement("index00");
-	index_00.setAttribute("font", m_index_0_0_data.value(Qt::FontRole).toString());
+	index_00.setAttribute("font", QETUtils::fontToString(m_index_0_0_data.value(Qt::FontRole).value<QFont>()));
 	auto me = QMetaEnum::fromType<Qt::Alignment>();
 	index_00.setAttribute("alignment", me.valueToKey(m_index_0_0_data.value(Qt::TextAlignmentRole).toInt()));
 	dom_element.appendChild(index_00);