wxGetKeyState() does not currently work on non-X11 GTK backends, and in some cases it has been reported to crash. It seems that the most likely use case for wxGetKeyState() is to query the modifier keys, so on non-X11 backends, use GTK+ calls to retrieve the modifier key state. Non-modifier keys are not currently implemented, update the documentation to mention this. Closes https://github.com/wxWidgets/wxWidgets/pull/320