Twenty Fourteen: make small-screen menu accessible to keyboard commands and voice-driven software by using a focusable `button` element rather than `h1` for the toggle element. Fixes #27147, props jartes and joedolson.

git-svn-id: https://develop.svn.wordpress.org/trunk@27594 602fd350-edb4-49c9-b593-d223f7449a82