Synchronization only keeps translations for first <file> container

[fabian-flassig]

Hi,
when having an XLIFF 2.0 document with multiple containers, the synchronization of translation units will only preserve existing translations for the first container, but not for the subsequent containers.
The XLIFF documentation clearly says that the use of multiple containers is allowed (http://docs.oasis-open.org/xliff/xliff-core/v2.0/os/xliff-core-v2.0-os.html#d0e1524).

I guess that e.g. xlf-documents.ts line 131 simply only considers the first node:
const fileNode = this.getNode('file', this.root);

A simple example is attached; synchronization of the de-DE file will show the effect.
localization.base.zip

Resolving this would be super helpful for larger projects!

[rvanbekkum]

Dear @fabian-flassig,
Thank you for filing an issue. I will have a look on how to approach that, but indeed I think that should surely be possible to support. 😉