Cleanup and unify keyboard input.

- Unify keycode values (secondary label printed on a key), remove unused hardcoded Latin-1 codes.
- Unify IME behaviour, add inline composition string display on Windows and X11.
- Add key_label (localized label printed on a key) value to the key events, and allow mapping actions to the unshifted Unicode events.
- Add support for physical keyboard (Bluetooth or Sidecar) handling on iOS.
- Add support for media key handling on macOS.

Co-authored-by: Raul Santos <raulsntos@gmail.com>

doc/classes/InputEventWithModifiers.xml

@@ -10,6 +10,12 @@
 		<link title="InputEvent">$DOCS_URL/tutorials/inputs/inputevent.html</link>
 	</tutorials>
 	<methods>
+		<method name="get_modifiers_mask" qualifiers="const">
+			<return type="int" enum="KeyModifierMask" />
+			<description>
+				Returns the keycode combination of modifier keys.
+			</description>
+		</method>
 		<method name="is_command_or_control_pressed" qualifiers="const">
 			<return type="bool" />
 			<description>