If an episode is being downloaded we setup a callback that will
supervise the widget and update it's state once the download action is
completed and the episode rowid is removed from `manager::ACTIVEDOWNLOADS`.
At first I tried creating custom InAppNotifications and send
them to the main loop to be added. That does not work sicne gtk
widgets are not thread safe. For now we can try having Action messages
that create them on demand. As oppose to create first then pass them.
