Expose window flags to Python

### 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](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Proposed new feature or change

Would it be possible to expose window flags to `O3DVisualizer`?

I would like to open window and set it as topmost. I see that the CPP Window implementation has these flags in [constructor](https://github.com/isl-org/Open3D/blob/28b4a0c137759f8618558c5fad0b4f7ddc308931/cpp/open3d/visualization/gui/Window.h#L36).

Also if there was ability to open the window maximized by default would be even better!

### References

_No response_

### Additional information

_No response_