implement spatial navigation

[jonathanzong]

We want to implement spatial navigation, which is where you navigate the x/y grid instead of the tree structure.

In our old prototype, we did this by adding a new section of the tree, but I think it's better to implement this as a table. The reason is, screen readers already have built in functionality to traverse html tables like a grid.

Old prototype:
https://mitvis.github.io/access-interaction/prototypes/jzong/random-access/

Instructions for old prototype:

• In the second layer, navigate to Grid view of scatterplot which is after all of the axes/legends
• down arrow to enter into grid view of scatterplot and use WASD to navigate

My partial attempt:
00d5222

Instructions for partial attempt

• press 'g' to bring up the grid menu
• there is a table. theoretically the buttons should open a table view of the data that matches the grid position