Describe the bug

CheckboxTree is almost completely unuseable with a screen reader, or a keyboard.

Reproduction steps

1. Attempt to expand / collapse nodes with a keyboard.
2. Try to read the contents of the tree view with the NVDA screen reader.

Expected behavior

The tree is completely operable via the keyboard, and reads correctly for screen reader users. See https://www.w3.org/TR/wai-aria-practices-1.1/#TreeView