Refactor undo/redo handling

[pcardune]

There are a few problems with this:

• [ ] undoableAction in the redux store is mutated directly. nothing in the redux store should ever be mutated.
• [ ] it doesn't handle the case when there is no "topmost action" to undo/redo very well. See the call sites for ReadonlyCMBEditor.getTopmostAction for examples.