Allow Designer to render Xaml when not wrapped in a suitable root.

[ghost]

Originally by @danwalmsley

i.e.

<TabItem xmlns="https://github.com/perspex" Header="Type">
  <DropDown Items="{Binding ProjectTypeOptions}" SelectedItem="{Binding ProjectType, Mode=TwoWay}" />
</TabItem>

Above could be rendered if Designer wrapped it in a Presenter.. currently it doesn't.

[ghost]

Latest Designer has this feature but testing is still needed.