Resolve "Handling of image.xml"

This MR

• updates the eXist-db version number to 6.0.1
• changes the implementation of finding out descendant URIs: where formerly a URI had the form of textgrid:{$URI} when the revision is 0, it is now always textgrid:{$URI}.{$REVISION} regardless of the URI value. This is important especially for the image.xml file where the image URI is always a full URI incl. revision.
• extends the function connect:update-image-xml to being able to handle the case where no new images are detected.

Closes #32 (closed)