Un-deprecate QMouseEvent::flags() and QGraphicsSceneMouseEvent::flags()

On one hand it looks like API clutter: a whole Qt namespace enum just
to track whether an individual mouse click is about to geenerate a
MouseButtonDblClick event. On the other hand, we should not remove it
without replacing it somehow, so that users don't lose the workaround for
QTBUG-25831 that it provides. That would be an invasive change because
this flags property exists in QMouseEvent, QGraphicsSceneMouseEvent
and in MouseArea { onClicked: doSomethingWith(mouse.flags) }

Reverts a small part of 4e400369c0

Task-number: QTBUG-25831
Change-Id: I9a3b4f6cc6b858012186f10ed57689f8c0f5fd79
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qevent.cpp

@@ -540,7 +540,6 @@ Qt::MouseEventSource QMouseEvent::source() const
 
 /*!
     \since 5.3
-    \deprecated in 6.0
 
     Returns the mouse event flags.
 

src/gui/kernel/qevent.h

@@ -307,7 +307,6 @@ public:
     QT_DEPRECATED_VERSION_X_6_0("Use globalPosition()")
     QPointF screenPos() const { return globalPosition(); }
     Qt::MouseEventSource source() const;
-    QT_DEPRECATED_VERSION_X_6_0("Internal, don't use")
     Qt::MouseEventFlags flags() const;
 #endif // QT_DEPRECATED_SINCE(6, 0)
 };

src/widgets/graphicsview/qgraphicssceneevent.cpp

@@ -653,7 +653,6 @@ void QGraphicsSceneMouseEvent::setSource(Qt::MouseEventSource source)
 
 /*!
      \since 5.4
-     \deprecated in 6.0
 
      Returns the mouse event flags.
 

src/widgets/graphicsview/qgraphicssceneevent.h

@@ -121,12 +121,8 @@ public:
     Qt::MouseEventSource source() const;
     void setSource(Qt::MouseEventSource source);
 
-#if QT_DEPRECATED_SINCE(6, 0)
-    QT_DEPRECATED_VERSION_X_6_0("Internal, don't use")
     Qt::MouseEventFlags flags() const;
-    QT_DEPRECATED_VERSION_X_6_0("Internal, don't use")
     void setFlags(Qt::MouseEventFlags);
-#endif // QT_DEPRECATED_SINCE(6, 0)
 
 private:
     Q_DECLARE_PRIVATE(QGraphicsSceneMouseEvent)