Support for Undo/Redo

[renaudpawlak]

Finally, I have found the time to add support for undo/redo :)

That's a major improvement and refactoring.

I have implemented a Command pattern, which can be seen the commands.js file.

Commands can be undone/redone through the menu or with the ^Z and ^Y shortcuts.

Please don't hesitate to report any regressions or weird behavior.