Project history (Undo button)

[jenncross]

This is a blue sky issue that I never thought we'd see resolved or have a chance at adding but with all the new fixes that @BrandonAPrice is doing, (Thanks!) I have enough hope to throw it into the ring...

Nielsen's design heuristics, on which the 2010 CLVP interface redesign was based, calls for supporting users in recovering mistakes... the traditional solution to this is an "undo button".

Since this software is for kids, and kids make mistakes (like that child who spends an hour putting all of the expressions they want into a sensor structure and then deletes the sensor structure ... losing everything), it would be AMAZING if we could record changes made to the expressions and sequences (even just preserving and undoing the last action would save so much stress for students, and the past 5 actions would be even better).

With the addition of the project files and the fix to #43 , I feel like this is withing reach. (Much like #43 having an undo for Sequences is way more important than for Expressions, though an Undo for expressions might help with Chris' "Emergency Stop" comment on #205 )

This is now done essentially done for the Sequence Builder in the Birdbrain Dev branch. If I have time later I'll also add it in for the expression builder.