curve/timeline: improved user interface behavior

* Implements "auto-fit" setting for curve view and sets it on open.
* Improves scroll zooming on all TimelineViewBase derivatives.
* Improves code sharing for better maintenance.

app/widget/curvewidget/curvewidget.cpp

@@ -169,6 +169,8 @@ void CurveWidget::SetInput(NodeInput *input)
   }
 
   UpdateInputLabel();
+
+  QMetaObject::invokeMethod(view_, "ZoomToFit", Qt::QueuedConnection);
 }
 
 const double &CurveWidget::GetVerticalScale()