timeline: use scroll cooldown when dragging playhead

app/widget/timeruler/seekablewidget.cpp

@@ -231,6 +231,7 @@ void SeekableWidget::mouseReleaseEvent(QMouseEvent *event)
   }
 
   dragging_ = false;
+  emit DragReleased();
 }
 
 void SeekableWidget::mouseDoubleClickEvent(QMouseEvent *event)

app/widget/timeruler/seekablewidget.h

@@ -78,6 +78,9 @@ public slots:
 
   virtual void TimebaseChangedEvent(const rational &) override;
 
+signals:
+  void DragReleased();
+
 protected:
   virtual void mousePressEvent(QMouseEvent *event) override;
   virtual void mouseMoveEvent(QMouseEvent *event) override;