trying to fix loadUi widget placement issue

[MagicRedDeer]

This is an attempt to fix #323. Where a UI loaded in PySide from a .ui file does not show up at the top of the parent widget as expected.

This solution creates a new widget as a child or the "base instance" and then uses the new widgets layout or central widget on the base instance.

[CLAassistant]

All committers have signed the CLA.

[mottosso]

Thanks for this. Could you implement a test for this as well? That way it'll get run during CI and ensure it works for all bindings on all platforms. Also if you could post a reproducible we could run locally as well, to confirm it works visually that would be great.

[mottosso]

Hi @MagicRedDeer, thanks again for submitting this PR. I know writing tests are a drag, but if you could paste something even in a comment, then we could run it on our end and confirm it works and doesn't break anything else. That would help this PR getting merged.