Add the ability to pull to refresh

[AnugrahJames]

Is your feature request related to a problem? Please describe.
While using Files many a times I've pulled down to refresh only to remember that it wasn't supported. I would love to have the ability to pull down than reach for the button. It would make Files much better for touch and gestures.

Describe the solution you'd like
The pull-to-refresh sample from the Xaml-Controls-Gallery would do it.
https://github.com/microsoft/Xaml-Controls-Gallery/blob/master/XamlControlsGallery/ControlPages/PullToRefreshPage.xaml

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