[Toolkits] Preserve toolkit scroll position when moving back a level

[microbit-matt-hillsdon]

We should ensure that we preserve toolkit scroll position when going back a level. It's quite irritating to be browsing and have it reset.

[microbit-matt-hillsdon]

We should hold off on this until the latest design changes have been implemented.

[microbit-matt-hillsdon]

We no longer have a third level so there's no point where the second-level scroll position is lost.