Trackbar size fixes

[BiGgUn]

Hi,

  I'm currently working on a patch about trackbar sizes fixes. The main goal
of this patch is to get rid off magic values and provide a more generic
approach of trackbar computation.
 Some test cases have shown for example that thumb length doesn't depend on
client rect fields. I also use some system metrics values (SM_CYCPATION) to
compute some trackbar's elements.

  I'll try to send the patch ( screenshots also) as soon as possible.

  Stephan