We were already using `gir -d` and especially now that our files are
separated across two directories that are relative to the directory
containing Gir.toml this only becomes cumbersome. Besides `gir` lacks
functionality to normalize the path, leading to ie.
gstreamer-gl/egl/sys/../../../gir-files in the version comment as a
result.
Make it an actual bitflags type instead of just an u64 alias and add all
the constants. As it's not defined as bitflags in C this needs to be
done manually.
Parameter mutability has been fixed (and reverted, hence overridden with
const=true)for this function in gstreamer; it can now be automatically
generated.
By setting the version to 1.8 and using `manual = true` an unconditional
trait implementation calling get_name is generated, while the
autogenerated version is omitted.
