Re: [w3c/editing] undo manager (#209)

The proposal seems to enforce a model where `undo` and `redo` are two seperate actions as evidenced by the distinct `UndoItemCallback`s on every `UndoItem`.
I usually consider both `undo` and `redo` actions as being the same generic function that defaults to a different stack and where every `UndoItem` is invertible. Here is some code to make this easier to understand:

```
const undoStack = Array();
const redoStack = Array();

function applyAction(originalStack, oppositeStack)
{
    const item = originalStack.pop();
    oppositeStack.push(invertAction(item));
    item.apply();
}

function undo()
{
    applyAction(undoStack, redoStack);
}

function redo()
{
    applyAction(redoStack, undoStack);
}
```

In my experience, it is easier to manage complexity when reasoning about invertible actions as opposed to implementing undo and redo as two separate actions. Should the `UndoManager` really push developers toward this direction?

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