Because the two independent Briar projects briar_repl and Briar GTK
combine forces, the api wrapper part that used to be part of Briar GTK
now becomes a new project on its own: python-briar-wrapper.

With this commit, everything specific to GTK and GNOME gets deleted.

For this to work, you have to put this project (GTK) and Android's
project into one directory and call the Android's folder
"briar-android".

https://reuse.software/

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

Parts of the changes are heavily inspired by GNOME Lollypop.