add ability to associate syle sheets with xml files.

For example: we have files of the type .ckl which is really an xml formatted file generated by the STIG viewer (DoD requirements), which the DoD provides an .xslt style sheet for.  I would be nice to be able to associate .ckl files with a registered xml style sheet in collaborator.  This would makes review and Markup, much easier.

To extend the functionality in a more generic way,  if a file was .xml file; and I had several sytle sheets registered for .xml files, if the default style sheet, was None, or styleA.xslt, during the review allow the user to specify a different .xslt either in the settings tab, or by right-clicking on the .xml file name and selecting a different style sheet.