PodcastDownloader changes

Changes:
Removes episode deleter out of PodcastDownloader
A separate PodcastDeleter
Indicator for podcast that have any downloads going
Ability to download multiple episodes at the same time
ToDo:
Ability to cancel a download


void PodcastDownloader::ReplyFinished() {
  Task* task = qobject_cast<Task*>(sender());
  list_tasks_.removeAll(task);
}
Should take care of it