Trackbar select range tweaks

[BiGgUn]

> Changelog:
> Oops, initial thumb length doesn't appear to depend on
> SM_CYCAPTION after all.

Thumb length DOES depend on GetSystemMetrics( SM_CYCAPTION) on a native platform. ( thumb_length = GetSystemMetrics( SM_CYCAPTION) +
2 [with no select range])
If wine uses standard values from windows there's no matter. But if a user decides to change these values in its registry, trackbars
won't reflect those changes in their sizes.

So i think that thumb length still need to depend on this metrics value.

Stephan

>     /* initial thumb length */
>     clientMetric = GetSystemMetrics(SM_CYCAPTION);
> ....