Can undo/redo "Unmark all".

Testing Done:
1. mark packages
2. mark other packages
3. click "Unmark All"
4. click "Undo"
5. You are back to step 2. (and not 1. as without the patch)
6. click "Redo"
7. You are back to step 3. (nothing marked and "Unmark All" button disabled)

REVIEW: 127546