add diff with time travel

[Delapouite]

No description provided.

[Delapouite]

Kind of implemented in 3cee2fc

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.