briar-headless.jar and python dependencies with flatpak-pip-generator.

Briar GTK now searches for the Briar Headless JAR at three places:
* _/app/share/java/briar-headless.jar_
* _/usr/share/java/briar-headless.jar_
* _~/.local/share/java/briar-headless.jar_

That jar got build from the most recent commit

briar@f61e2b39

of the merge request adding the `lastChatActivityKey`

briar!1242

This wasn't possible before because my version of flatpak was buggy.

This is a change not-yet upstreamed but currently to be reviewed at
briar!1242.

Builder didn't want to compile the project as a flatpak project.

* https://code.briarproject.org/briar/briar/-/wikis/changelog#anchor-127
* https://code.briarproject.org/briar/briar/-/tags/release-1.2.7
* https://media.dorfbrunnen.eu/briar/jars/009/

Fixes #13

Developed by Purism at https://source.puri.sm/Librem5/libhandy

Documentation at https://honk.sigxcpu.org/projects/libhandy/doc/

briar@9ebe49b8

Although I find YAML a lot better, GNOME Builder sadly doesn't support
it: https://gitlab.gnome.org/GNOME/gnome-builder/issues/520

This essentially reverts 5fc6b517, but
also updates some dependencies.

Inspired by
https://code.briarproject.org/grote/briar-cli-python-demo/blob/8ced7b9f37e6ee59dac6042c9bcf21c54c648e9e/briar-cli.py.

With help of https://github.com/flatpak/flatpak-builder-tools

	./pip/flatpak-pip-generator requests2

With help of `python -m json.tool`.

There might have been a bug in my Arch Linux installation of GNOME
Builder. I've now installed it the recommend way:

	flatpak install --from https://flathub.org/repo/appstream/org.gnome.Builder.flatpakref

Fixes #1.

https://gitlab.gnome.org/GNOME/gnome-build-meta/tags

This uses a new compiled jar based on the current `headless-stdin`
branch:
* https://code.briarproject.org/briar/briar/tree/3f6e5fe098a45977458e9f717418692105b81c61
* https://media.dorfbrunnen.eu/briar/jars/002/briar-headless.jar

Based on current master + clikt 1.7.0

Modules can't contain slashes and class names seem to follow CamelCase
style.

That's a binary prebuilt by me. Later, we should build it by source.

I would like to use v11 as v10 is already deprecated, but it fails when
building with that newer version.

Based on a template generate by Gnome Builder.