Add nix environment and update script

[jtojnar]

Nix package manager makes it easy to obtain all the typelibs without having to install them globally. Just run nix-shell (or enter the project directory if you have direnv) and then you will have the libraries available in the environment.

Then you can run tools/update.sh to re-generate all stubs.

[jtojnar]

Just using this for local development.

[jtojnar]

Tried regenerating all stubs that worked (GstPbutils generation segfaults) before and after #167 for comparison.

Only two stubs are invalid code (Gst #172 and Gtk3 #169) but that was a pre-existing condition.

[lovetox]

you need to use the -o argument, not output to a file.

The current stubs need to be parsed to find all override comments.

[lovetox]

Can you contain the nix stuff to some subfolder, maybe with a README how to use?

Further please separate this MR from updating the files.

[jtojnar]

I mostly just opened this to share the diff for #167

It probably does not make sense to merge the Nix environment unless someone else is interested in using it.

[lovetox]

I would keep the update.sh though, it makes it easier for people to execute all steps, they can comment out the libs they dont want to update.

[matperc]

I love the idea of update.sh.

IMHO we should automate this to run every X days, to keep every stub in sync with upstream

[jtojnar]

Opened #174 with the update script.