show all existing artwork types for a specific media type in the "Choose art" dialog

This commit implements what @jmarshallnz thought was already there in the "Choose art" dialog. It retrieves all known artwork types for a specific media type from the video database using the new GetArtTypes() method and adds them to the list of artwork in the "Choose art" dialog independent of whether they have a valid value or not. This means that when I (manually) add a "landscape" artwork type to tvshow A, "landscape" will also show up in the "Choose art" dialog for tvshow B but with no image assigned to it. That should make it easier for users to add the same custom artwork types across all media items of the same type.