CTK/Slicer UI customization with stylesheet

Hey @Sam_Horvath @lassoan @pieper @jcfr @cpinter

I just wanted to bring to your attention this draft PR

Reference:

visual DICOM browser (and in general CTK) uses Qpalette to change colors dynamically. This does not go well when customizing CTK/Slicer by styleSheet. Probably we should use the styleSheet approach everywhere. For example, the background yellow warning on filters does not work when applying a custom styleSheet. Discuss with Sam for a good design. Ideally the solution should:

  1. use one unique style file containing the stylesheet for all the CTK classes
  2. change color of qt widgets in CTK dynamically with the Property Selector, e.g.: Dynamic Properties and Stylesheets - Qt Wiki
  3. custom Slicer apps should be able to change the style by simply rewriting and reloading the style file