Implement undo/redo

[ThomWright]

Fairly low-priority at the moment, but should be fairly trivial using redux middleware.

Look at this? - https://gist.github.com/gaearon/2b3ee315bb6a3b280b7c

[ThomWright]

In the state object, views contains:

• Static state - component types, positions etc.
• Transient state - current and voltage

The 'transient state' changes during each main loop cycle.

The 'static state' should be undoable, i.e. each change added to a history stack. Moving the transient state out would help make this possible.

For example:

{
  views: {
    id: {
      typeID,
      props: {
        id,
        dragPoints: [Vector], // 2 drag points
        connectors: [Vector],

        resistance|current|...
      }
    }
  },
  NAME_ME: {
    id: {
      currents,
      voltages
    }
  }
}

This still leaves the problem of adding/moving components - history stack should be updated at the end of adding/moving, not during, even though the views state will change a lot during moving.