Don't pass 'Application' around anymore. Store it in a global instead.

The idea is that there can't be more than one active UI anyway. There is
only one stdin and stdout. Passing it around everywhere is way too
cumbersome and this made the code much cleaner in many regards.

For pymux, where we'll need multiple applications, we'll find a
work-around. Maybe by setting the application each time right after
receiving data.