Fix ctkDICOMobjectListWidget memory leak and other enahancements

* Replaced QSlider with ctkSliderWidget

* Added copy file path to the clipboard.

* changed the order of columns in dicom table to: Tag, Attribute, value, VR, Length

* Double-click on tag table item would open the browser and search the tag in dicomlookup.com

* other stylistic changes and code refactoring.