ENH: Fixed 2498 and other ui enhancements for dicom browser.

This patch would fix
http://na-mic.org/Mantis/view.php?id=2498
and also fix the UI issues which Csaba reported on 2014-04-24 12:17 here:
http://na-mic.org/Mantis/view.php?id=3578
and fix a bug in saving the disabled plugins in Slicer application settings