Implement #108961 - there's now a shortcut and window-specific setting
for 'Block global shortcuts', which allows applications like Xnest etc.
to get shortcuts like Alt+F4 etc. that'd be otherwise intercepted as
global shortcuts. A shortcut turn it on and off globally, window-specific
setting only for the given window, the shortcut can turn it temporarily
off even for window with a window-specific setting.