GtkCodeGen: Add gir files to repo (if allowed), and regenerate bindings in CI to ensure properly generated #259
Description
Currently, when a contributor adds additional bindings to GtkCodeGen, there's a high likelihood that they'll end up with a bunch of unrelated changes due to the generator using the contributor's own gir files (from their Gtk installation). This isn't ideal, because it can lead to backwards compatibility bugs being introduced when people have newer gir files that I haven't built the generator to handle properly (gtk is hard).
I should add my gir files to the repo (if Gtk's licensing allows it), and add a CI job to regenerate the bindings from source and make sure that they match the contents of the PR. The regeneration serves both to avoid a malicious contributor from hiding stuff in big generated blobs the reviewers may assume have been correctly generated, and to ensure that contributors don't forget to regenerate and format the bindings when they update GtkCodeGen.