ViewContainerLayout: remove moveWidget override

[colin-grant-work]

What it does

Fixes a bug that breaks ViewContainerLayout.insertWidget by redundantly swapping items in the _widgets array.

How to test

To Confirm Bug

1. Remove second (main) commit from this PR.
2. Open the application and open the Navigator widget.
3. Run the command Bug: Try to swap top two widgets in Navigator several times.
4. The first invocation may work, but subsequent invocations will fail, and the widgets will stay in the same order.

To Confirm Fix

0. Bring back the second (main) commit from this PR.
1. Run steps above. All invocations should take effect, and the drop shadows on ViewContainerParts should be correct (none on top, shadow between subsequent views)
2. Drag and drop widgets in ViewContainers.
3. Behavior should be as on master, and drop shadows should be correct.
4. With items in non-default positions, refresh the application.
5. Positions should be restored correctly, and drop shadows should be correct.

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

[vince-fugnitto]

I confirmed the bug on master, and that the change successfully fix the issue 👍
I did not notice any regressions when testing.