Mark QApplication::fontMetrics as obsolete

It returns the QFontMetrics of the desktop widget, so just for the
default screen. Any usage of it is probably wrong, esp since code should
use QFontMetricsF instead.

Change-Id: I0b4b85a74a2c9b6cc023ffda0b2b399f9fc3c1c4
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/kernel/qapplication.cpp

@@ -1559,7 +1559,9 @@ QWidget *QApplication::activeWindow()
     return QApplicationPrivate::active_window;
 }
 
+#if QT_DEPRECATED_SINCE(6,0)
 /*!
+    \obsolete Use the QFontMetricsF constructor instead
     Returns display (screen) font metrics for the application font.
 
     \sa font(), setFont(), QWidget::fontMetrics(), QPainter::fontMetrics()
@@ -1569,6 +1571,7 @@ QFontMetrics QApplication::fontMetrics()
 {
     return QApplicationPrivate::desktop()->fontMetrics();
 }
+#endif
 
 bool QApplicationPrivate::tryCloseAllWidgetWindows(QWindowList *processedWindows)
 {

src/widgets/kernel/qapplication.h

@@ -100,7 +100,11 @@ public:
     static QFont font(const QWidget*);
     static QFont font(const char *className);
     static void setFont(const QFont &, const char* className = nullptr);
+
+#if QT_DEPRECATED_SINCE(6,0)
+    QT_DEPRECATED_VERSION_X_6_0("Use the QFontMetricsF constructor instead.")
     static QFontMetrics fontMetrics();
+#endif
 
     static QWidgetList allWidgets();
     static QWidgetList topLevelWidgets();