Add correct accessibility features

[stowball]

The W3C describe in detail how a tree view component should be written to be accessible here: https://www.w3.org/TR/wai-aria-practices-1.1/#TreeView

Here are the paraphrased missing features:

• Missing aria roles: tree, treeitem, group
• Missing aria states: aria-expanded, aria-multiselectable, aria-selected
• Checkboxes shouldn't be real checkboxes
• Each <li> should have tabindex="-1" unless it is the first aria-selected item, then its tabindex is 0 (this stops keyboard users from having to tab through all of the items) to get past the tree view
• Up/down should move up/down the nodes
• Left/right should expand collapse parent nodes
• Space should (de)select a node
• Enter should (de)select a node, unless it is a parent, in which case it expands/collapses

It's worth reading the aforementioned authoring guidelines to make a component that is accessible to all users.

I understand it will be quite a bit of work, but it's very important that all users, regardless of disability and device can use this control (and every control on the web).