Separate gir-files into submodule and gst-gir-files

Following https://github.com/gtk-rs/gir/pull/1041 it is finally possible to use gir-files as submodule (without copypasting xml files and risking to get out of date!) while at the same time having our own Gst-specific files in a separate directory.

Perhaps we should move these files into their own gst-gir-files repo as well for sharing/reuse and to have better hashes in version.txt. Alternatively gir should omit hashes for folders that are contained in the base/core/root repo and not in a submodule.

Note that this change makes it impossible to jump back to older branches unless manually deleting the folder (submodule) at ./gst-files and reinstating it when checking back out on a branch with these changes. Don't think that little inconvenience during this transitioning phase is worth putting it under a different name though.