Add QEventPoint::pressTimestamp() and priv setPressTimestamp()

There was no good reason for QEventPoint::pressTimestamp() to be
missing.  The case for QEventPoint::timestamp() is a bit dubious
because it's redundant with QInputEvent::timestamp(); but for now,
we keep m_timestamp, in anticipation that Qt Quick may need to keep
copies of eventpoints between events, thus they need to avoid depending
on their shorter-lived parent events too much.

Change-Id: Iec38acfdfaa2afb3dc77d5cd1b95baa8d301c0fd
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/gui/kernel/qevent.h

@@ -154,6 +154,7 @@ public:
     State state() const { return m_state; }
     int id() const { return m_pointId; }
     QPointingDeviceUniqueId uniqueId() const { return m_uniqueId; }
+    ulong pressTimestamp() const { return m_pressTimestamp; }
     qreal timeHeld() const { return (m_timestamp - m_pressTimestamp) / qreal(1000); }
     qreal pressure() const { return m_pressure; }
     qreal rotation() const { return m_rotation; }
@@ -177,7 +178,7 @@ protected:
     QVector2D m_velocity;
     QPointer<QObject> m_exclusiveGrabber;
     QList<QPointer <QObject> > m_passiveGrabbers;
-    ulong m_timestamp = 0;
+    ulong m_timestamp = 0; // redundant with m_parent->timestamp(), but keeps timeHeld() working in a saved copy
     ulong m_pressTimestamp = 0;
     QPointingDeviceUniqueId m_uniqueId;
     int m_pointId = -1;

src/gui/kernel/qevent_p.h

@@ -88,6 +88,8 @@ public:
 
     void setTimestamp(const ulong t) { m_timestamp = t; }
 
+    void setPressTimestamp(const ulong t) { m_pressTimestamp = t; }
+
     void setState(QEventPoint::State state) { m_state = state; }
 
     void setUniqueId(const QPointingDeviceUniqueId &uid) { m_uniqueId = uid; }