Retain unknown attributes on Sentence Save

Following PerseusDL/perseids_docs#31 we want to be able to support
additional markup/attributes in treebank files, even if the editor
doesn't support curation of these.

Before this commit a new sentence was built from scratch by using
nodes right out of the editor session when a sentence was saved, now 
we update the attributes we know of in place of the old document.

WARNING: This is not mergeable yet, waiting for approval that this is in 
fact a good way to solve this - I am very open for suggestions on how to 
do it better! 
After such a confirmation I'd have to update the `treebank-putsentence-local.xq`
as well.