Accessible Builderius menu

Makes the Builderius menu (the sidebar of templates, pages, components and admin links) usable from the keyboard and a screen reader. The menu button announces that it opens the menu, focus moves into it when it opens, and it is exposed as a tree: the arrow keys move between the collapsible category headings and their items, the right and left arrows open and close a category, Enter or Space toggles a heading or opens an item, Home and End jump to the ends, and Escape (or the panel’s Close button) closes the menu and returns focus to the button.