Handle back button more gracefully

[MondayHopscotch]

It currently is either ignored entirely (if 'disable back button' is selected in the options menu) or it exits the app entirely.

It should behave the same as the back buttons that are in the app.

[MondayHopscotch]

These are all handled by changing the scrollItemScreen:

• ~World Select~
• ~Level Select~
• ~Palette Select~
• ~Replays~
• ~Stats~

These will have to be handled separately:

• ~Credits~
• ~In-game pause~
• ~Options screen~
• Title screen (should game exit? Should confirm exit? Going to break this into its own story)