def add_state(self, state, widget):
self._remove_state(state)<EOL>self._add_state(state, widget)<EOL>self._repack_and_regroup_children()<EOL>
Add a state to the deck.

The given widget will be allowed to fill the entire space available to 
the deck, meaning that the widget's ultimate size is determined only by 
its claim and its alignment.  Typically every widget in the deck will 
be the same size, to allow for smooth transitions between states.  If 
the given state already exists, it will be overwritten without error.