Reduce item pane message font size

E.g., "Loading items…"

Maybe we want this bigger, but this is roughly what we had before and
matches the tag selector and item pane messages.

chrome/skin/default/zotero/overlay.css

@@ -158,6 +158,7 @@
 	position: absolute;
 	top: 0;
 	left: 0;
+	font-size: .7em;
 }
 
 .items-tree-message div {