#685 Undo does not record insert/override mode for redo

[Lim Wei Zhong]

Steps to reproduce:

1. (Start Dev-C++)
2. Ctrl-N (Open new document)
3. asdf (Type some text)
4. Insert (Enter override mode)
5. Ctrl-Home (Position cursor to start of document)
6. qwer (Type some more text)
7. Insert (Exit override mode)
8. Ctrl-Z (Undo)
9. Ctrl-Shift-Z (Redo)

Outputs: qwerasdf
Expected output: qwer