CTK/Slicer UI customization with stylesheet

Hey @Sam_Horvath @lassoan @pieper @jcfr @cpinter

I just wanted to bring to your attention this draft PR


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