Add updater using Onova

[Zopolis4]

Got tired of updating manually, so I added an updater using Onova.

[JohnVeness]

Personally I'd like to see MPF available in "app stores" like Chocolatey, Scoop, Flathub etc., rather than self-updating.

[Zopolis4]

That's a matter for the package managers to handle, and not one that precludes the implementation of update support in MPF itself.