Attempt to refactor as a step towards moving gstreamer integration in…

[andrew-otiv]

…to a separate crate.

[andrew-otiv]

Discussion also here: https://github.com/slint-ui/slint/discussions/4623

[tronical]

I think it's a good idea to clean up this code and make it more modular. Feel free to proceed. This is currently marked as a draft, not ready for review yet. Also, it seems that the formatting check fails - I suggest to run cargo fmt :)