vtkCocoaGLView.mm: Fix macos KeyCode and KeySym
- Recover KeyCode and KeySym without modifiers when invalid without it
- KeySym fallback to "None" instead of nullptr
- Use Latin1 ASCII for support of non-US layout and cross platform consistency
Related to: https://discourse.vtk.org/t/about-keycode-keysym-modifiers-crossplatform-and-reliable-code/12598
Edited by Mathieu Westphal (Kitware)