Poster downloading update

Poster downloading update

Remove the thumbnail variants of the poster downloader's Fetch and the
poster factory's queue methods, instead add width and height parameters.
The IMDb poster downloader now downloads the small poster image directly
from the title page when only a thumbnail is requested. The full poster
images are scaled down to 288x400.
The poster factory now creates thumbnails from the downloaded posters
(nearly) according to the freedesktop.org thumbnail spec: they are stored
as jpeg files and the sizes of normal and large thumbnails are optimized
for the list view and poster grid, respectively.