fix(PropertiesPanel): only call callbacks once on first render

[marstamm]

with #225 we introduced changes so the layout reacts to external changes.

With the changes, we introduced another re-render cycle during bootstrapping, causing layoutChanged and descriptionLoaded to be called twice.

This PR ensures the callbacks are only called once.

[marstamm]

9600411 and a1cf5c2 fix the root-cause of #227.

This is ready for review

[nikku]

Isn't this equivalent to an effect without dependencies, i.e.

useEffect(() => {

}, []);

[marstamm]

It's actually the exact opposite 😄
useEffect(fn, []) is only executed once, ONLY during the first render.
useUpdateEffect is executed every time a dependency changes, EXCEPT on the first render

[nikku]

💡