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 remaining from Briar GTK gets modified to
match python-briar-wrapper's needs.

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.

Based on a template generate by Gnome Builder.