nsIMicrosummaryObserver Interface

 Called when an observed microsummary updates its content.
 Since an observer might watch multiple microsummaries at the same time,
 the microsummary whose content has been updated gets passed
 to this handler.
 XXX Should this be onContentUpdated?

 @param microsummary
        the microsummary whose content has just been updated

void onContentLoaded(in nsIMicrosummary microsummary)

 Called when an element is appended to a microsummary set.
 XXX Should this be in a separate nsICollectionObserver interface?

 @param microsummary
        the microsummary that has just been appended to the set

void onElementAppended(in nsIMicrosummary microsummary)

 Called when an observed microsummary encounters an error during an update.
 
 @param microsummary
        the microsummary which could not be updated

void onError(in nsIMicrosummary microsummary)