Allow secondary documents in transforms. Add ghost:sourcefile attribute to img elements

[fsteimke]

See Issue #598

Changes the signature of fp:run-transforms in docbook.xsl to return a map(*) instead of a document node.

The implementation may seem over-complicated. I started with the obvious approach: only one map within the xsl:iterate for the principal result and secondary results. Which worked fine for most of the tests, but failed without any error message at table-cals.049, which is a huge table. I can only suspect that there is a problem with very large values inside maps in saxon under special circumstances? So i have to use two parameters within xsl:iterate: a document-node for the principal document, and a map for secondary result documents. Both are merged into a single map as function result on completion.

I also added a ghost:sourcefile to image files. My main motivation for secondary results in pipelines are copy instructions for media files.