lua : add film handling

darktable.films : pseudo table of films

film.id  : the film's id
film.path : the film's path
film.# : images belonging to that film

image.film : the associated film object
lua : new event post-import-film

trigered when a film has finished import.

one extra parameter : the film that was imported
lua : make database.import return usefull information

when imorting an image : the image that has been imported

when importing a directory : the film that is imported

note that the function returns immediately, so when importing a film, the object is correct but the images havn't been imorted yet

use the post-import-film event to find out when import is done