Add functionality to switch player for rebinding

With the new change_player() method in KeyBindingList and an extra
conditional in SettingsState's get_player_input(), the Key Binding
display will now be updated whenever the "Set Key Bindings for..."
Setting has its value changed.
Allow dynamically-updated bindings in title_state

The player1_keys and player2_keys attributes in TitleState are demoted to
method variables within get_player_input(). They directly reference the
key binding lists with the StatePass, allowing changes to the bindings
from the Settings Screen to have an effect on this State as well.