Clear window attention if all messages are marked read

Fixes #758

// FREEBIE

js/conversation_controller.js

@@ -62,6 +62,9 @@
             storage.put("unreadCount", newUnreadCount);
 
             setUnreadCount(newUnreadCount);
+            if (newUnreadCount === 0) {
+                window.clearAttention();
+            }
         }
     }))();