Use shared generator in `gir` repository

Depends on https://github.com/gtk-rs/gir/pull/1097: draft until this is finalized and merged.

The generator (the script that invokes gir for all Gir.toml crates) is now shared betweem 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.