Show and hide a split view pane with mvvm

[Nemonek]

What does the pull request do?

This pull request adds two samples to the repository. Both shows how to open and close a SplitView's Pane with MVVM. One does so by using Reactive UI, while the other does not use any framework, just the INotfyPropertyChangedInterface.

Scope of this PR:

• [ ] fix or update to an existing sample
• [x] add a new sample

What is the current behavior?

Checklist

If this is a new Sample

• [ ] Added a ReadMe-file
• [ ] Updated the landing page
• [ ] Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with a link to your sample in the related documentation page

In any case

• [ ] Spell-checking done
• [ ] Checked if all hyperlinks work
• [ ] Checked if all images are visible

Fixed issues