Add landmark elements and regions for navigation

[microbit-robert]

Also:

• fixes an issue where extra and seemingly unnecessary vertical scroll is present on the data samples page due to the icon picker.
• fixes icon picker positioning

Regions as of this PR:

• header with the app-wide actions
• main containing the table-like area
  • named action regions
• "data samples toolbar" or "testing model toolbar" containing the page specific actions (intentionally avoided the word action here)
• live graph

We think the live graph makes sense as a separate region as the other actions aren't related to it and are different per page. It's also something we talk about a lot in docs etc.

See #517

[github-actions]

Preview build will be at
https://review-createai.microbit.org/regions/

[microbit-robert]

Converting to draft as we need the regions for each action on the testing model page as well, which will hopefully improve the speed of navigation to the recognition point input.

Edit: Now done.

[microbit-matt-hillsdon]

LGTM, we'll get Kirsty's feedback on this and iterate if needed but let's get it in as a first cut.