Undo all of a multiline import at once

[raineorshine]

• Blocked by Changes are not persisted after undo #1631
• Blocked by 40f1a7c (revert needed)
• Blocked by importFiles refactor

Steps to Reproduce

- a
- b
- c

1. Create a new, empty thought.
2. Paste the above thoughts.
3. Undo.

Current Behavior

The last imported thought is removed.

- a
- b

Expected Behavior

All imported thoughts should be removed.

The state should reverted to right before the paste.

- [empty thought]

There is already some logic in place to group navigation actions together into a single undo. A similar approach should be applied here.