Add Undo feature to Metric and Segment Builders

[#1]

Working with complex segments and calculated metrics that contain many nested containers and may take a few screens, it's quite often when I move or add something into a wrong place. Sometimes the recovery may take longer than making everything from scratch.

The Undo feature (Cmd+Z, Ctrl+Z) would be extremely helpful if supported in the Metric Builder and Segment Builder UI.

[#2]

Literally had this issue today.   Accidentally deleted a container which contained loads of logic - no undo, no way to get it back