Dont create pages' data-context until navigation

[Xiaoy312]

What would you like to be added:

   at Uno.Gallery.Sample.CreateData(Type dataType)
   at Uno.Gallery.Sample..ctor(SamplePageAttribute attribute, Type viewType)
   at Uno.Gallery.App.GetSamples()
   at Uno.Gallery.App.AddNavigationItems(NavigationView nv)
   at Uno.Gallery.App.BuildShell()
   at Uno.Gallery.App.OnLaunchedOrActivated()
   at Uno.Gallery.App.OnLaunched(LaunchActivatedEventArgs e)

We are currently instantiating all data-contexts, that we ever need, the moment we are launching the app...
This should've delayed until the navigation actually occurs; build one as we need, and not the whole batch.

Why is this needed: general performance

For which Platform: any

Anything else we need to know?

This should also fix the problem of, data-context being shared between navigation.
Would this may sound correct, it is NOT the behavior we expect.

Additionally, we should also check if themes' and toolkit's weren't suffering the same fate.