[containments/panel] Fix initial sizing

On startup the panel's containment size is effectively undefined. It then stays that way for 150ms until a timer fires.

Ultimately the timer breaks layouts internal usage of QQmlParserStatus. For runtime changes layout internally most relayouting is buffered internally till the polish event which is once per frame.

Removing this makes startup faster, less glitchy and more declarative.

Testing done: apitrace trace plasmashell --replace starts with the system tray in the right place, only task manger icons animate, and even those look better.