add diff with time travel

[Delapouite]

Kind of implemented in https://github.com/byteclubfr/copycast/commit/3cee2fcb5a061506928f8dd08f3983602bffd20a

But not really the optimal solution.

The socket emits the full tree at each change. We keep the last 5 received client side. A slider on the top right of the screen let the user go back in time or keep synchronized with last revision.

[naholyr]

We may store versions based on datetime instead of a count, whenever the trainer presses Ctrl+S every half second :P

Maybe even regroup close changes into one single change.