Presence normalization for media containers

The same accessing methods (as the one used by MediaGroup: nbTotalMedia() and nbPresentMedia()) should be used for all abjects representing a group of medias:

• Folder
• Album
• Artist
• Genre

mentioned in merge request vlc-android!887 (merged)

I haven't changed the Folder accessor since the media are always present when the folder is, a nbPresentMedia would either be 0 or equal to nbMedia, so I'm not sure it makes sense to expose.

Otherwise this is fixed by !371 (merged)

assigned to @chouquette

closed with merge request !371 (merged)