Add/use the renpy.object.Sentinel class.

This is a class that can be used to create sentinel objects. Even if
we pickle and unpickle an object of this class, it remains the same
object, as far as the 'is' operator is concerned.

This fixes a problem with the shift+R module backups. (The same
problem 9dec1f32ab7cac643534ae551a5bc1b8462dfb90 tried to fix.)