Hide items pane at startup to avoid showing column lines

Column groups aren't processed until after startup progress meter closes, so
otherwise lines show behind the progress meter for all possible columns.

chrome/content/zotero/zoteroPane.js

@@ -365,6 +365,8 @@ var ZoteroPane = new function()
 		
 		yield Zotero.unlockPromise;
 		
+		// The items pane is hidden initially to avoid showing column lines
+		document.getElementById('zotero-items-tree').hidden = false;
 		Zotero.hideZoteroPaneOverlays();
 		
 		// If pane not loaded, load it or display an error message

chrome/content/zotero/zoteroPane.xul

@@ -371,6 +371,7 @@
 								onselect="ZoteroPane_Local.itemSelected(event)"
 								oncommand="ZoteroPane_Local.serializePersist()"
 								flex="1"
+								hidden="true"
 								zotero-persist="current-view-group">
 								<treecols id="zotero-items-columns-header">
 									<treecol