R1:3f5109b66864

In prose diff dialogs (like "Show Details" in transactions), show "old", "new" and "diff" tabs

Summary:
Ref T7643. When you do something like this:

- Edit a task description.
- Click "Show Details" on the resulting transaction.
- Get a prose diff dialog showing the change.

...now add some "Old" and "New" tabs. These are useful for:

- reverting to the old text by copy/pasting;
- reading just the new/old text if the diff is noisy;
- sometimes just nice to have?

(This looks a little rough but…