Make selection highlight follow first responder and update on focus changes

- Choose emphasized/unemphasized color based on key window and text view
  first responder state (shouldUseEmphasizedColor)
- Refresh selection and current-line highlights on become/resign first responder
- Centralize color updates via updateBackgroundColor(), including on
  effective appearance changes and window key changes
- Avoid stale emphasized selection when window is key but text view not focused
- Minor refactor: move selection colors to file-level constants