[Steps]
1. Create an Impress doc
2. Apply "Title, Text" layout to the slide, and type several paragraphs in the
Text text-box in slide. Switch to layout viewmode, "Insert" -> "Expand Slide".
3. Press Undo button.
4. switch to Normal viewmode.

[Result]
1. In step 3, the view in Slides bar back to one slide, but in Outline view,
undo has no effect.
2. In setp 4, after switch to Normal viewmode, the slides still expanded.

[Expectation]
1. Ecpanded slides undo successfully.