Regression: Undo is less reliable

[schanzer]

1. Start the editor with the standard sample file.
2. Switch to blocks mode.
3. Drag the multiply expression (line 13), and drop it on the -5 inside the if-expression (line 15).
4. Hit Ctrl-Z

EXPECTED: the drag operation is undone
ACTUAL: nothing

There are other examples where this fails (deleting an expression and then hitting undo(), for example), but I suspect that they are all symptoms of the same problem.

[schanzer]

Poked at this today - it looks like undo actually does the right thing, but the BlockEditor fails to re-render to show the transformed blocks. In BlockEditor::handleChange(), actionFocus is being retrieved as undefined. As a result, no action is fired and there is no call to commitChanges()