Need to be able to alter escape key behavior

### Checklist

- [x] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [x] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [x] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### Proposed new feature or change

In the visualization.gui (and probably the older visualization implementations as well), the escape key's default action is 'end program' with no warning.

And as far as I've been able to determine, there's no way to override that: it's possible to capture the keystroke (and possibly issue a warning), but the script terminates regardless of the return value from the callback. Losing work is probably the worst thing a GUI can do, and it's far too likely to happen with this behavior.

(Feel free to escalate this to a bug report, but it appears to be working as designed, no matter how wrong that appears to me.)

### References

_No response_

### Additional information

_No response_