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.
This removes the empty widget from the ShowStack. The plan is
to have a ShowsView which will be the parent of ShowStack and
an Empty Widget. Then make ShowStack have only valid populated
states of ShowsPopulate, ShowWidget and AboutEpisodeWidget later.
