File Directory Treeview design updates

[smhigley]

From the 3/16 APG call, the plan is to sort out the following:

File Treeview w/ Computed Properties

(link: https://w3c.github.io/aria-practices/examples/treeview/treeview-1/treeview-1a.html)

• Figure out the simplest possible realistic example of a treeview
• Figure out what selection means within this tree (with focus or not), and add appropriate attributes
• Look into updating the "File selected" text box, perhaps by displaying a fake file pane instead

File Treeview w/ Declared Properties

(link: https://w3c.github.io/aria-practices/examples/treeview/treeview-1/treeview-1b.html)

• Come up with a realistic example of a treeview where declared props are needed
• Implement virtualization, probably
• Maybe multiselect?