The generator (the script that invokes `gir` for all `Gir.toml` crates)
is now shared between all projects that utilize `gir` to simplify
collaboration, instead of having to copy-paste improvements back and
forth.  All GStreamer-rs specifics (path to `gst-gir-files`) has been
taken care of by using `girs_directories` in `Gir.toml` instead like
`gtk(4)-rs`.