Add and position blocks from toolbox with keyboard

[tracygardner]

Make adding and placing blocks from the toolbox fully accessible and intuitive with keyboard controls.

• Expand subcategories when you navigate to them with the keyboard (they auto expand when you click/tap)
• When you press T you end up in the search box instead of the first category
• Esc should close flyout (and return focus to Toolbox category if that's where it came from)
• Toolbox visual highlights are unclear
• Scrollbar appears on toolbox, can we avoid this
• Is Tab the right way to get to the search results flyout?

[tracygardner]

Expand subcategories on keyboard focus to make them discoverable.

1bc5c06