Restore full functionality of ImageInfo.

Consequently cache all fields (including rating, tags of course), use lazy loading everywhere.

Remove tags paths method, code outside libs/database may use the AlbumManager with its cache please, it's faster. Only supply (cached) tag ids.

Remove unused constructor.

Do not cache DatabaseUrl, but construct on demand from ids and image name (album name is cached in ImageInfoCache, TODO change notification, album root is retrieved from CollectionManager)

Add method to retrieve ImageComments object for sophisticated comments storage.

TODO: copy method, change notification