replace timeout by await displayed

This solution:
https://github.com/nmearl/ipygoldenlayout/blob/ccda4edd11a2187c65deba1d3476660dd795c4d9/src/widget.ts#L95
Can be done more elegantly:
https://github.com/maartenbreddels/ipymaterialui/commit/a50cacf9bc6412c93a08b3406e8d00e24b1982f7#diff-ea5a23bdd5334b8c7af57d11b36b00e4R22