Refactor App, removing #refresh, #next_key, #missized? .

Remove `App` methods which were wrappers around `Display::Screen`
methods and only called once. Change ordering of methods.

HACK: Skip `App#refresh_loop` loop if no `@screen`; can't decide if
this only happens in certain test scenarios, or whether there is a valid
runtime scenario similar to that in `App#command_loop`.