Add converter for dpi to pixel font size

Add a little function used to convert if needed the size of a font set
in dpi to pixel. Because the conversion from pdi to pixel can't
be exactly identical, the text size in the diagram can be a little
different, in the other side the switch between screens with different
dpi is no more a problem.

sources/utils/qetutils.h

@@ -31,6 +31,7 @@ namespace QETUtils
 	QString marginsToString(const QMargins &margins);
 	QMargins marginsFromString(const QString &string);
 	qreal graphicsHandlerSize(QGraphicsItem *item);
+    void pixelSizedFont (QFont &font);
 
 	bool sortBeginIntString(const QString &str_a, const QString &str_b);