Sensible default key displays + allow users to override key displays at the App level (#1213)

* Get rid of string split key display

* Include screen level bindings when no widget is focused

* Add default key display mappings

* Allow user to customise key display at app level

* Better docstring

* Update CHANGELOG.md

tests/snapshot_tests/test_snapshots.py

@@ -118,3 +118,9 @@ def test_css_property(file_name, snap_compare):
 def test_multiple_css(snap_compare):
     # Interaction between multiple CSS files and app-level/classvar CSS
     assert snap_compare("snapshot_apps/multiple_css/multiple_css.py")
+
+
+# --- Other ---
+
+def test_key_display(snap_compare):
+    assert snap_compare(SNAPSHOT_APPS_DIR / "key_display.py")