Automatically download/update ARTist

[schrnz]

Given that we have ready-made ARTist versions at https://github.com/Project-ARTist/ARTist/releases, it would be nice to directly download the correct ARTist version into ArtistGui instead of requiring it to be compiled into the app as an asset. Effectively, this allows to distribute a slim apk on platforms where there is an ARTist release available.

We should still keep around the possibility for developers to work with self-baked versions so that those have a higher priority than the downloaded ones (if needed). Maybe control this via a settings switch?