Bug in text undo

[diomidov]

1. Switch to text tool.
2. Create a text object with text A
3. Create another text object with text B
4. Switch to select tool and back to text tool (you can't skip this step for some reason)
5. Click on A
6. Press Ctrl+Z
7. The text has changed to say B

Tested in Chromium.

It looks like different text objects (sometimes?) share the same undo stack.