Also listen on other enter button and empty entry after sending.

Co-Authored-By: ialokim <ialokim@t-online.de>

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

GTK isn't thread safe. Therefore, I was experiencing race conditions.

Reading Briar's output isn't much reliable.

https://reuse.software/

This is a squash merge of `complete-revision`.

Parts of the changes are heavily inspired by GNOME Lollypop.

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