Set each extraData object on event queue under its respective itemID

This relates to the following discussion on zotero-dev:

https://groups.google.com/forum/#!msg/zotero-dev/UF7BIjVqaJw/6HtDEQlU4isJ

From the code in notifier.js, it appears that modify events are meant to carry the old data of objects being modified in their extraData object. A key mismatch was causing the object always to be empty. This change should pass the data through, for both single-item and multiple-item modify events.

(Pull request refiled. The original pointed at a much earlier point in branch development for some reason.)