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

sandbox/darren/screens_focus.py

@@ -0,0 +1,47 @@
+from textual.app import App, ComposeResult, ScreenStackError
+from textual.binding import Binding
+from textual.screen import Screen
+from textual.widgets import Static, Footer, Input
+
+
+class Focusable(Static, can_focus=True):
+    pass
+
+
+class CustomScreen(Screen):
+    def compose(self) -> ComposeResult:
+        yield Focusable(f"Screen {id(self)} - two")
+        yield Focusable(f"Screen {id(self)} - three")
+        yield Focusable(f"Screen {id(self)} - four")
+        yield Input(placeholder="Text input")
+        yield Footer()
+
+
+class ScreensFocusApp(App):
+    BINDINGS = [
+        Binding("plus", "push_new_screen", "Push"),
+        Binding("minus", "pop_top_screen", "Pop"),
+    ]
+
+    def compose(self) -> ComposeResult:
+        yield Focusable("App - one")
+        yield Input(placeholder="Text input")
+        yield Input(placeholder="Text input")
+        yield Focusable("App - two")
+        yield Focusable("App - three")
+        yield Focusable("App - four")
+        yield Footer()
+
+    def action_push_new_screen(self):
+        self.push_screen(CustomScreen())
+
+    def action_pop_top_screen(self):
+        try:
+            self.pop_screen()
+        except ScreenStackError:
+            pass
+
+
+app = ScreensFocusApp(css_path="screens_focus.css")
+if __name__ == "__main__":
+    app.run()