Backends now have to send input to the given `chan::Sender<Event>`.
They send these events from a separate thread, allowing selection
between input and callbacks.
This currently breaks the BearLibTerminal backend, which requires all
calls to come from the UI thread.
This might not be super-safe for the ncurses backend also. We hope that
input and output are separate enough that they can run concurrently without
problem.
