Navigation/MD pages: pop all but first on select

[d-loose]

Clears the navigator stack of the right pane in YaruLandScapeLayout and YaruNavigationPage when (re-)selecting an entry in the left pane.

See https://github.com/ubuntu-flutter-community/software/issues/254

Edit: This requires us to use the navigator provided by the respective Yaru page, rather than nesting another one below.

Pull request checklist

• This PR does not introduce visual changes, or
  • I ran flutter test --update-goldens and committed the changes if there were any, or
  • I added before/after/light/dark screenshots if the visual changes I made were not covered by golden tests.

    	Before	After
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    Dark

[Feichtmeier]

Thanks @d-loose !

The the yaru widgets example does not have a nested navigation I tried your fork and branch in the software app and it does not work for me after clean, upgrade and get

Edit: This requires us to use the navigator provided by the respective Yaru page, rather than nesting another one below.

Ah! So we need a second change in for example software?

[d-loose]

The the yaru widgets example does not have a nested navigation

I'll add an example page and a test.

I tried your fork and branch in the software app and it does not work for me after clean, upgrade and get

Edit: This requires us to use the navigator provided by the respective Yaru page, rather than nesting another one below.

Ah! So we need a second change in for example software?

Yes - with this approach we only have access to the navigator provided by the Yaru page itself. But for the software store that's not really a problem, since all the navigators there only contain a single page, so the required changes are minimal (I'm not even sure why they're there, probably from a time when the yaru pages looked different :D)

[Feichtmeier]

@d-loose ok, thanks !