vtkXRenderWindowInteractor: Fix keycode and keysym
in !8740 (merged) , specifically, 30a89b03, keycode was incorrectly modified to always ignore modifiers from the keyboard layout.
- Fix it by restoring previous logic
- Fix widgets relying on the new behavior
- Improve other widgets where modifiers may have an impact
- Fix unsafe GetKeySym usages
- Improve documentation
Edited by Mathieu Westphal (Kitware)