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.
Added Option for changing tables text density in dicom browser

stylistic changes
Merge pull request #448 from mehrtash/ctk-dicom-object-list-widget-enhancements

ctkDICOMobjectListWidget enhancements
Merge pull request #438 from mehrtash/3579-very-few-patients-visible-in-DICOM-Browser

Add Combobox for changing the density of dicom table views.