Fix view not updating when Model is changed.
Fix view not updating when Model is changed.

Consider the following binding

```
plotView.SetBinding(PlotView.ModelProperty, "PlotModel");
```

This updates the model when the property is changed in the ViewModel. However these changes are not reflected in the View. When adding `SetNeedsLayout` after the Model is set in the code. The changes now reflects in the UI.
oxyplot/oxyplot-xamarin