Allow Pluggable XSLT support in XML Export

The current RSS support limits the fields that you can export. It is after all only intended to provide users with an RSS view of the issues. However, it would be useful if the existing XML link would allow you to get a comprehensive dump of the currently visible issues in XML, and select an XSLT transform to be applied to the XML. The use case would be something like this:

(1) User opens the issue navigator, performs appropriate filtering, then clicks the XML link.
(2) Popup appears allowing the user to select from one of the registered XSLTs.
(3) The selected transform is then applied to XML.

The default XSLTs might include GanttProject, and MS Project XML file formats.

Making it pluggable makes it possible for administrators to add their own XSLTs to support additional export formats. These might include transforms that take JIRA issues and generate requirements documentation in XHTML or other formats.