Split translation file of Briar GTK and its metadata

In order to being able to focus more on the translation of Briar GTK's core components, I though about splitting up the translation file into a Python/UI part and a metadata part.

We currently have 90 strings to translate for Briar GTK, and roughly half of it are coming from its metainfo file. This file is used by programs like GNOME Software to generate a beautiful application page. However, especially the changelog part I don't see as critical as the core program and therefore suggest to split it up. It's just a matter of changing one line of code in tools/generate_data.sh and doing some configuration in Transifex. Since I'm now a maintainer there, I can do this all on my own.

Once we have things split up, we can start to prioritize core components of Briar. I suggest to make the Android and GTK parts "Urgent", the manual "High" and GTK's metadata "Normal".

CC @akwizgran @grote