implemented cursors in the timeline

Note: this does not use the cursor SVGs in app/ui/cursors since I was never
really satisfied with them. They're kept for posterity for the time being.

app/widget/timelinewidget/view/timelineviewmouseevent.cpp

@@ -74,6 +74,11 @@ void TimelineViewMouseEvent::SetEvent(QEvent *event)
   source_event_ = event;
 }
 
+const qreal &TimelineViewMouseEvent::GetSceneX() const
+{
+  return scene_x_;
+}
+
 void TimelineViewMouseEvent::accept()
 {
   if (source_event_ != nullptr)