Add support for fixed time brightness fading

"_ex" as in "extended". Typically used when new parameters need to be added to existing api function, something like

foo(x) -> foo_ex(x,y) { ... } and foo(x) { foo_ex(x, 7); }

In this case the evolution of code led into original function dropping out altogether, so the "_ex" is kind of vestigial organ. Since it is "not to be used directly" kind of helper function, "nc" for "nonclient" or "pv" for private might be more appropriate