WIP: Actix-Qt integration

Integrating the Qt event loop into an Actix system enables a lot cleaner code to communicate between the UI and the main system.

• Fix UI rendering
• Make sure that sending a QEvent can safely re-enter the Rust object. Maybe even find a way to statically assert this?
• Vigorously test that closing the UI actually stops all of Tokio and Actix.
• Write a blog post! https://www.rubdos.be/corona/qt/rust/tokio/actix/2020/05/23/actix-qt.html
• This yields more work to #25 (closed)