Dropbutton should report open/closed state to assistive technology

Problem/Motivation

The drop-button UI widget does not report the open/closed state to assistive tech like screen readers. The more-actions button has a good text alternative to let the user know what to expect, but after pressing it there is no confirmation about what happened.

Proposed resolution

Use ARIA states and properties to describe the dropbutton behaviour to assistive technology APIs.

• Update dropbutton.js to report the open/closed state, using aria-expanded, together with aria-controls.
• NO, this isn't appropriate for our existing uses of dropbutton. See #14.

Remaining tasks

• Decide on the markup/DOM pattern. Does this require any markup changes? Would be great if we can implement this without changing any CSS selectors.
• Write a patch
• FunctionalJavascript tests - operate the more-actions button, and assert that the aria state attributes have changed.

User interface changes

Accessibility improvements, principally benefiting screen reader users.
No visual design changes.

API changes

None expected.

Data model changes

None expected.