fix sensitivity and state of 'hide read threads'

fix sensitivity and state of 'hide read threads'

This fixes this problem:
'Thread View' is set and 'Hide read threads' is set.
Unset 'Thread View', 'hide read threads' is greyed out but still set,
and 'hide read messages' is greyed out also.

With this fix, when 'Thread View' is unset, 'hide read threads' is
automatically unset too, thus 'hide read messages' is selectable.