Optionally make the Back button an Undo

Add a settings option to make the back button become "Undo". This toggles 
the behaviors of the back button and "undo" menu item. When selected, the 
menu button option that is usually "Undo" will instead be "Exit Game" or 
something similar.

This behavior only applies when playing an unfinished game. If the game is 
complete or the user is on any other screen then the back behaves normally.

Use case #1: When a mistake was made several moves ago repeatedly pressing 
"menu" / "undo" is tedious.

Use case #2: When a new user presses the back button expecting to preform 
an Undo they are returned to the list of games. Upon re-entering their game 
their undo history is lost.

I would also recommend making this the default behavior, but for new 
installations only.

Original issue reported on code.google.com by shepp...@gmail.com on 3 May 2010 at 7:38

harshal261090 / opensudoku-android

Optionally make the Back button an Undo #90