Don't call UI functions from helper code

This may seem much more complicated but actually is much cleaner. Add each
thread we start to the list of future results and add a new UI function
that updates the UI once all of the threads have finished.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qthelper.cpp

@@ -870,6 +870,4 @@ void learnImages(const QDir dir, int max_recursions, bool recursed)
 	}
 
 	QtConcurrent::blockingMap(files, hashFile);
-	if (!recursed)
-		DivePictureModel::instance()->updateDivePictures();
 }