xP: highlight hovered buffer list items

To make it apparent which one would be closed by a middle click.

xP/public/xP.css

@@ -120,6 +120,9 @@ button:hover:active {
 .item.activity {
 	font-weight: bold;
 }
+.item:hover {
+	background: #f8f8f8;
+}
 .item.current {
 	font-style: italic;
 	background: #eee;