At the moment if I want to search history and know which results are bookmarked I have to do the search query, then do bookmark searches. Instead the results returned from the history query should just have a `bookmarked` flag (or by some other name). The http://mdn.beonex.com/en/XPCOM_Interface_Reference/nsINavHistoryResultNode.html docs say there is a `bookmarkIndex` attribute which I think we can use here to determine if a result is bookmarked and avoid extra queries.