The table button can become a menu button when the cursor's position
in the editor is within a table. So we'd need to update the button with
ARIA attributes appropriate for a menu button.
This is best done when the button's highlight gets toggled, so we're
adding an event listener for when this happens and add/remove the ARIA
attributes accordingly.

Including efficiency improvements from https://github.com/davosmith

* Menu button fix
  - Added aria-haspopup, aria-controls, and aria-expanded attributes.
* Menu fixes
  - Added aria-labelledby that points to the menu button label.
  - Removed the dialog role in the menu's container.
  - The ul tag needs the "menu" role.
  - The li tag needs the role "none" instead of the "presentation" role

Reference:
https://www.w3.org/TR/wai-aria-practices-1.1/#menubutton