Add support for XML references accross multiple XML files

[tbazant]

XML references work fine within a single XML file. Please, add support for the scenario when a.xml's <xref/> references b.xml's xml:id
thanks!

[angelozerr]

Indeed it would be nice but it requires to scan folders and could parse a lot of XML files.

I had in my mind to provide the capability to define a search scope to avoid scannings all files from the project.

It is not a trivial issue.