Uno Gallery: Add capability to upload a Material.json file like for ColorPaletteOverride for showing the controls in a kind of live preview

[DevTKSS]

I did scroll a bit trought the uno gallery and seeing the material area there I had the Idea, if it might be possible, to enable a upload for the runtime of the gallery, so we could not only see the definitions on the excample controls, then also see them in the style we would get them in the actual application.