Indicate favorites status in menu

[CDrummond]

In the browse view, the menu always has "Add to favorites", even if item is already in favorites. Perhaps before (or whilst?) showing the menu, a check should be performed. If the item is in favorites, change it to a remove from favorites entry. The favorite status should be cached with the item to save repeated lookups.

This should be handled after #23. Using CometD will allow material to receive notifications when favourites have changed.