AGScopeBarGroup tweaks

- Added -[AGScopeBarGroup selectedItemIdentifiers]
- Added -[AGScopeBarGroup setSelected:forItemWithIdentifier:]
- Changed how the group popup menu items are added. They now are removed when the menu closes. I've forgetten the precise reason for this change, but I believe this is because if an AGScopeBarItem's menuitem contains a view (which could require a chunk of memory for backing) it would have meant loading all views for all menu items in the group whether the menu was open or not. This makes it more efficient.