Added "Compare..." functionality to the xmlviewer script that allows users to compare the current document with another ODT document using kdiff3.

* kdiff3 is used to show the diff between the content.xml, styles.xml, etc. files of the current ODT as displayed+saved by KWord and those one that got loaded (or just totaly another one).
* this should make it a bit easier to see what changed between a full load+save circle ;)