Add redo feature

[hakash]

Expand upon Add undo( #12 ) and add the feature to redo after an undo. Depending on the implemented solution in #12 this might be as easy as pushing current state to a different stack before applying the undo. triggering a redo would typically reverse this by one step. Performing an action that alters state should invalidate the entire redo-stack, as that data is now out of date.