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


Don't use `shared_ptr` unless you absolutely have to. Just delete the `Task` when you remove it from the list and use `qDeletaAll()` in `PodcastDownloader::~PodcastDownloader`