This commit reimplements support for the empty view in the ShowStack.
The current structure is the following:
* A Content stack which holds the HomeStack and the ShowStack.
It's what is used in the headerbar StackSwitcher.
* The HomeStack is composed of the HomeView and an EmptyView.
* The ShowStack is composed of the PopulatedStack and an EmptyView.
* The PopulatedStack is composed of the ShowsView and the ShowWidget
currently. An AboutEpisode widget is planned to be added here also.
