Fixes #414, it is possible to edit the "extra" field and another field simultaneously

(As noted in the ticket, this bug applied to clicking between any fields (not just "extra") without changing a value -- it also kept the second textbox from focusing properly on click.)

Not entirely sure how I broke this, but here's a fix.

chrome/content/zotero/itemPane.js

@@ -864,6 +864,9 @@ var ZoteroItemPane = new function()
 	
 	function showEditor(elem)
 	{
+		// Blur any active fields
+		_dynamicFields.focus();
+		
 		//Zotero.debug('Showing editor');
 		
 		var fieldName = elem.getAttribute('fieldname');