Improve index look-up (closes #139)

Hardens the item look-up in the index files.

Before the hardening, #139 (closed) occurred because there were two files in $ixp:dataCollection which held the same xml:id, causing the look-up to throw an exception. With narrowing down the files in which the look-up is performed to the actual index files, we can be sure that we have no double ID entries (this is ensured in the TextGridLab).