remove time machine headings and fix keyboard controls

[riknoll]

fixes microsoft/pxt-microbit#6863

removes the unnecessary h3 tags on the time machine dialog and fixes keyboard controls for the react-common tree component.

here's the new keyboard controls:

• pressing right on a collapsed subtree causes it to expand without changing focus
• pressing right on an expanded subtree causes focus to move to the first item
• pressing left on an expanded subtree causes it to collapse without moving focus
• pressing left on a tree item or collapsed subtree will focus the parent treeitem if it exists
• up, down, home, and end all navigate the visible items like you'd think

also fixes the roles for the treeitems and the groups, simplifies the exported react elements, and makes it so that only the selected tree item serves as a tab stop in the page tab order