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


Doesn't need a translation, it's used to generate a proper metadata for files on disk, this files might be used by other devices so using "Podcast" without a translation is a safest solution