Accessibility: Interacting with the site tree using the mouse should set roving tabindex

[GuySartorelli]

#2068 adds a roving tabindex to the site tree.

As per silverstripe/developer-docs#866 our accessibility guidelines for roving tabindex will state that mouse interaction that would ordinarily set focus should set focus, and should update the roving tabindex.

Acceptance criteria

• Interacting with the site tree with the mouse in a way that would ordinarily set focus on the relevant element (e.g. clicking a checkbox in batch actions mode) does set focus in that treeitem
• The same interaction updates the roving tabindex, so the treeitem interacted with has tabindex="0" and all other treeitems have tabindex="-1"