The `keycode` field of `InputEventKey` is supposed to be "unshifted";
That is, what the key would output if no modifier keys were pressed.
This should match what's written on the key label, but `Key` enumerates
also all keypad keys, which require a modifier. We thus require some
extra checks for them.
Note that this can still allow "stuck keys", but that's an even deeper
problem.
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