ocrd_sbb_textline_detection used the output XML by main.py as is, and
– by doing this – threw away any input data from the input PAGE,
including the critical pc:AlternativeImage and the less important
pc:MetadataItem.
Fix this by merging the segmentation results into a file created from
the input file.
Also add a pc:MetadataItem processingStep about the segmentation
operation.