Navigator keyboard tree

Makes the Navigator behave like the WordPress list view: the arrow keys move through the elements (the canvas selection follows), the right arrow opens a branch and steps into it, the left arrow closes it and steps out to the parent, and Home and End jump to the first and last. The same keys navigate the canvas while it is in selection mode, including when the Navigator panel is hidden. The tree is exposed to screen readers as a proper tree, so each element announces its level, whether it is expanded, and its position.