Bug 829820 - Ability to mark selected SMS thread as Read/Unread

I think you should control the read/unread button status right in this ```selected.selectedCount``` ,and the read/unread button status checking could be simplified into:
```
var hasUnreadselected = selected.selectedList.some((id) => {
  var thread  = Threads.get(id);  // Get thread data from Threads. We don't prefer to information from DOM.
  if (thread && thread.unreadCount) {
    return thread.unreadCount > 0;
  }
  return false;
})
// And set readUnread.dataset.action depends on hasUnreadselected, we don't have to know the actual count.

```
I haven't tried this, it's just a rough idea about the implementation.