Don't reload Playroom on every story change

[rbardini]

Playroom doesn't seem to update its code editor on URL hash changes. That's why the addon panel assigns the code-appended URL to the iframe key prop, so that React recreates the element and reloads Playroom whenever new code should be used:

storybook-addon-playroom/src/Panel.tsx

Lines 61 to 63 in f638ed1

	return (
	<Iframe key={url} allowFullScreen src={url} title="Playroom" />
	);

Ideally, we should be able to update the iframe's src alone and Playroom would pick up the new code. This may require changes to Playroom though.