[libdanbooru/danbooruservice] Move thumbnail downloading to a lambda

slot

This greatly reduces complexity and prevents the ugly passing of
properties along the jobs.

src/libdanbooru/danbooruservice.h

@@ -96,7 +96,7 @@ namespace Danbooru {
         QSet<QString> m_blacklist;
         DanbooruPost::Ratings m_maxRating;
 
-        unsigned int m_currentPosts; // To tell when to quit
+        unsigned int m_postsToFetch; // To tell when to quit
 
         KImageCache* m_cache; // Pixmap cache