fix(material-toolbar): accessibility improvements

To add structure to the page, toolbar components should include heading tags. Previously, `material-toolbar` contained `<span>` elements that did not contribute to the overall page outline. In addition, any items with `ng-click` should be accessible from the keyboard. With these changes, users can apply `ng-click` to an anchor element  in `material-toolbar` and see a visual style consistent with the rest of the docs.