Accessible builder tabs

Wires the builder’s tab strips as proper tab lists for screen readers and the keyboard: the persistent template and component tabs above the canvas, the settings panel’s Content and Styles tabs, and the Navigator’s Elements, Selectors and CSS vars tabs. Each tab announces whether it is current and each strip becomes one Tab stop where Left and Right, Home and End move between tabs; Enter or Space switches to the focused tab. Delete closes a focused canvas tab through Builderius’s native close action, and the new-template button receives an accessible name.