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


https://github.com/clementine-player/Clementine/blob/master/ext/libclementine-common/core/closure.h#L151

It's used all over clementine.