remove dt_film_import_blocking

it was lying about what it was doing, this does not change the fact that the lua API is not working like expected, but it works like it used to, so  fix in another commit

also fix DT_SIGNAL_FILMROLLS_IMPORTED not being triggered at the right time
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
usermanual: some first steps towards 1.4

main changes: reorganize docs on darkroom with a large section to explain
general concepts; first documentation on drawn masks.

still a lot to do!
Merge branch 'master' of ssh://github.com/darktable-org/darktable
usermanual: a few more screenshots
usermanual: some additions
Allow blend modes to be translated differently
updated pt_PT translation
usermanual: more rework
Merge branch 'master' of ssh://github.com/darktable-org/darktable
usermanual: on combined masks