Prevent Output widget from handling any drag/drop events

[colin-grant-work]

What it does

Fixes #10932 by preventing the Output widget from interfering in its parent's handling of drag-drop events.

How to test

1. Try dragging and dropping widgets onto the Output widget.
2. Observe that the behavior is the same as dragging/dropping on top of other widgets: you can can drop in the middle to put a new widget on the same tabbar, or to one side / top / bottom to split the panel.
3. Ensure that you don't see the behavior from the second cast here where the Output widget accepts the drop of the new widget as though it were an output channel.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

@kittaakos, I'd be happy to hear your thoughts about this approach.

[kittaakos]

I have checked out the branch and verified the changes locally; I can confirm that it works.

• I could not drop an editor into the Output widget (✅, Ref: GH-7347: Added scroll-lock to the Output view #7570 (comment)), and
• I could split the view when the Output widget was on the top (✅, Ref: Widget drag-drop fails over Output widget #10932).

verify_output.mp4

Thank you for the cleanup!

[msujew]

I can confirm that dragging widgets over the output view now behaves as expected 👍