Navigating between top-level stacks

[BeksOmega]

This discussion is about navigating between top-level block stack when you're already at the top level. See also the related discussion about skipping to a different top-level stack when you are within a stack.

There are several different ways we could handle this, which work better or worse for different sets of our personas:

1. Two keys to move between top level stacks.
   This might be particularly good for our limited motor control Emily persona, and our blind Alex persona. But annoying and limiting for our low vision Maria persona and RSI Jason persona.
2. Directional keys to move between top level stacks. E.g. the right arrow key to move to the next stack to the right, or the down arrow key to move to the next stack below.
   This might be particularly good for our low vision Maria persona and RSI Jason persona. But more difficult for our limited motor control Emily persona, and our blind Alex persona

[BeksOmega]

This is also related to the discussion about navigating blocks in the toolbox.