We need a way to get the keys that are available on the keyboard currently
in use (e.g. does the keyboard have a numpad separator key, a mouse wheel,
a middle mouse button, an @ key on the primary layer).

The motivation for providing this capability is bugs such as: 4342183,
4342186, and probably others.

The root cause of 4342183, etc., is that behavior is undefined when you
try to press a button that does not exist. Perhaps we should throw a runtime
exception when this happens. It would be up to clients to get the capabilities
first, or deal with the exception.