Twenty Nineteen: Consider modifying top-level menu icons when the touch menu is active

[laurelfulford]

From feedback from @melchoyce in the Twenty Nineteen GitHub repo:

When the touch menu is active, it'd be cool to try adjusting the top-level expand icons to point right instead of down (or be replaced with a plus icon or something?).

There's a bit of cognitive dissonance that happens when you click a down arrow, and the panel comes in from the side. It's a little disorienting at first.

Original ticket: ​https://github.com/WordPress/twentynineteen/issues/563