Add a user-configurable shortcut that allows breaking out of an active keyboard-shortcut-inhibit session

If an app uses the keyboard-shortcuts-inhibit protocol, and also pointer confinement, there's no way for the user to re-enable the global shortcuts, or even transfer focus to another window.

xfwl4 never disables the ctrl+alt+backspace kill-compositor shortcut, or the change-VT shortcuts.

There should be another shortcut, user-configurable, that allows the user to disable (and re-enable) an active shortcut-inhibit session, in order to allow the user to escape buggy or misbehaving (or actively malicious) apps.