- add a method to add a group of thumbnails to the thumbnail load thread.

This is meant for a future implementation, where the view pushes all currently visible thumbnails that need to be loaded to the _front_ of the queue. Currently, new tasks are always appended and wait for older ones. Prepending is not an option to keep the order of item loading unchanged.

- remove the one-second sleep, to real sleep. Audited that condvar is waked whereever needed.

Though, bugs may come up...