Fix: handling for works encoded with mei4+ in getLinkTarget (#491)

## Description, Context and related Issue
I extended the XPath to make shure that work titles of MEI4+ files can
also be found.

Closes #337 

## How Has This Been Tested?
eXist-db 6.2.0 by manipulating data in the database.

## Types of changes
- Bug fix (non-breaking change which fixes an issue)