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 <[email protected]>