UWP NavigationService.Close not working for pages in SplitView
## 🐛 Bug Report

I'm using SplitView in my UWP app (It is NavigationView to be detailed). I'm loading pages in Content frame of SplitView using MvvmCross navigation service and attribute on Views: 

`[MvxSplitViewPresentation(SplitPanePosition.Content)]`

Structure is something like: 
NavigationPage(Displayed in RootFrame) -> Page1(SplitView Content Frame) -> Page2(SplitView Content Frame) -> Page3(SplitView Content Frame)

When I'm on Page3 I have button there binded to MvxCommand in VM which makes call to NavigationService.Close(this)

I expected to go back to Page 2 but instead nothing happend. Am I doing something wrong?

I checked code of MvxWindowsViewPresenter and it looks like closing SplitView will never work. There is a method called when closing is performed:
https://github.com/MvvmCross/MvvmCross/blob/13eb483cd737a22b65e1e7d1b97e96b277b9a642/MvvmCross/Platforms/Uap/Presenters/MvxWindowsViewPresenter.cs#L142
It just calls ClosePage method and ClosePage method looks like cares only about top frame _rootFrame and not about nested SplitView Content frame:
https://github.com/MvvmCross/MvvmCross/blob/13eb483cd737a22b65e1e7d1b97e96b277b9a642/MvvmCross/Platforms/Uap/Presenters/MvxWindowsViewPresenter.cs#L185

So it found my NavigationPage as a CurrentView and when checks if argument viewModel is equal ViewModel set on page it obviously is not equal and ClosePage does nothing instead logging warn: Ignoring close for viewmodel - rootframe's current page is not the view for the requested viewmodel.

Is there another way to close page (Go back) displayed in content frame of SplitView?
My current solution is overrided CloseSplitView method in my custom presenter derived from MvxWindowsViewPresenter. But I would expect it should be implemented in framework.

```
protected override Task<bool> CloseSplitView(IMvxViewModel viewModel, MvxSplitViewPresentationAttribute attribute)
{
	if ((_rootFrame.Content is IMvxWindowsView) && (_rootFrame.Content is Page currentPage))
	{
		var splitView = currentPage.Content.FindControl<SplitView>();
		if (splitView == null)
		{
			return Task.FromResult(false);
		}

		Frame nestedFrame = null;
		if (attribute.Position == SplitPanePosition.Content)
		{
			nestedFrame = splitView.Content as Frame;
		}
		else if (attribute.Position == SplitPanePosition.Pane)
		{
			nestedFrame = splitView.Pane as Frame;
		}

		if (nestedFrame == null)
		{
			return Task.FromResult(false);
		}

		if (!nestedFrame.CanGoBack)
		{
			return Task.FromResult(false);
		}

		nestedFrame.GoBack();
		return Task.FromResult(true);
	}

	return Task.FromResult(false);
}
```

### Expected behavior
Possibility to go back through pages shown in SplitView Content frame
### Reproduction steps

- UWP App with a page with SplitView control as a start page and some pages loaded into Content part of SplitView. 
- On each page button to go to next page and button to go to previous page.
- ViewModels previous button command calls NavigationService.Close(this) 

### Configuration

**Version:** 7.0.1

**Platform:** 
- [ ] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [x] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms