GSoC Time Capsule

Adding simple undo / redo functionality into Catroid. This basically means that user mistakes in the scripts, sounds, costumes shall be un- or re-doable.

Here according to the feedback given in https://github.com/Catrobat/Catroid/pull/878 by @stypen I have created a new branch and implemented a test to test the undo redo functionality in Sound Fragments. 

Like @stypen said, I think Command Design pattern is appropriate to implement this time capsule feature and to keep the code clean. So I tried to start the implementation after getting me familiar with that design pattern.