Move focus logic to screen, add more key replacements, collapse bindings in footer (#880)

* Move focusing logic to the Screen level

* Update tests to support per-screen focus management

* Some additional key name replacements

* Improve rendering of bindings in footer when multiple items have same action

* Clean up footer, allow key_displays csv

* Prevent exception when widget is not in screen

examples/dictionary.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import asyncio
-from typing import Any
 
 try:
     import httpx