Avoid unnecessary UI updates in various situations

beginUpdateBatch() doesn't suppress select events, so various batch
operations were triggering lots of extra selects.

This hopefully fixes the long hang some people were seeing when making
changes with an open quick search (which I was only able to reproduce in
Standalone).
Expose launchNonNativeFiles hidden pref in Firefox

"Open PDFs and other files within Firefox when possible"
Make X-Zotero-Version and RIS/Refer prefs Firefox-only
Don't update item when leaving an empty field

It looks like this has been happening for at least all of 3.0, including
when tabbing between empty fields.
Fixes #47, Dragging attachment to item removes search results
Fix "results.sort is not a function" in tags autocomplete

This occured if there were no autocomplete results for an entered tag
string.