Undo/redo

I have started to implement a demo of my GSOC 2014 project idea which is "Adding a (time capsule like) history to Catroid". This is only implemented to undo/redo the deletion of sound files. 

1. Go to sounds fragment.
2. Add few sound files.
3. Delete one or two sound files.
4. Hit Undo/Redo menu icons. You can see the deleted sound files are showing in the list. You can hit the redo icon and delete the sound files as well.