Refactoring of the key input system.

- Registry which contains the list of key bindings
- Clean up of existing key bindings.
- Key binding manager: utility for managing Vi and Emacs bindings
- Rename of the `Line` class to `Buffer`.
- Added filters.py: for enabling/disabling part of the system at
  runtime.
- Added a focus_stack, a tool for redirecting the input from one buffer
  to another.