Template10: Update NavViewEx

[JerryNixon]

There are several things wrong with it, yet it is close.

• [ ] Get it to build, there are some things
• [ ] Update it to use Nuget T10 packages
• [ ] Update the sample app.
• [ ] Update the .nuspec file
• [ ] Upload to Nuget.org for Windows-XAML

[jp-weber]

I want work on point 1-3, because I have already implemented this in large parts of my fork. For me, it makes sense, if we use WinUI library for the NavigationView.

https://github.com/jp-weber/Template10/tree/master/Source/Template10.Extras.16299/Controls

In the sample project I am looking for better ways to set the header in a simple way. We could add all header templates to the shellpage resource list and set them as HeaderTemplates after navigation or loading. With a naming convention you could set here e.g. 'XHeaderTemplate'.

For the individual pages, e.g MainPage.xaml.cs you could now add the ViewModel of the page as (ShellPage).Header content, so that commands from the ViewModel can be added to the template.

[JerryNixon]

One thing to consider is that I would like to break this out to its own Nuget package. I also think it might make sense to consider how (if) we can decouple it from Template10 so it would be useful outside of using the framework. Still not sure. Also, I only care about Template10 2 support.