Support merging with disassociated source namespace

[NebelNidas]

Currently, if a tree has

nsA	nsB
cls1NsAName	cls1NsBName

and you want to merge

nsB	nsC
cls1NsBName	cls1NsCName

cls1NsCName never finds its way into the tree, as the visit pass doesn't supply a nsA name. Even worse, if you visit with nsC on the source and nsB on the destination side, an exception is thrown, since the incoming source namespace (nsC) doesn't exist in the tree yet.

Both of these cases can be solved by iterating all incoming namespaces that actually are present in the tree (nsB in this case), searching for entries with the same name (here: cls1NsBName) and if found, merging the new data into that existing entry, leading to the following result:

nsA	nsB	nsC
cls1NsAName	cls1NsBName	cls1NsCName

That's exactly what this PR does, however there's an edge case I wasn't sure how to handle: What if we find a match in more than one namespace, but each one belonging to a different entry?
Tree:

nsA	nsB	nsC
cls1NsAName	someNsBName
cls2NsAName		someNsCName

To be merged:

nsB	nsC	nsD
someNsBName	someNsCName	someNsDName

This would match both cls1 and cls2. As of now, this PR returns after merging with the first match, but we could of course merge with the remaining matches too or throw an exception instead. What are your opinions on this?