Add bindings for libgstplayer
Main trickyness here is that it's still unstable API. We should only support the latest stable release here (1.12 at this time).
There are multiple steps here:
- Add a Gir_Gst*.toml and autogenerate all relevant types, see https://github.com/gtk-rs/gir/
- Make sure the autogenerated code is correct, add manual code as needed
- Write manual bindings for anything that can't be automatically generated (macros, miniobjects, ...)
- Add src/lib.rs for re-exporting, Cargo.toml, etc