Use ARIA disclosure pattern for Toolbar buttons with trays

Problem/Motivation

1. The relationship of toolbar buttons to their trays is not conveyed to assistive technology.

   The Toolbar previously attempted to convey this using aria-owns, but it was discovered that his usage was incorrect in this issue: #3066954: Admin toolbar not usable with latest versions of JAWS due to mis-use of aria-owns, and aria-owns was removed in order to address a bug impacting screenreader users. The correct ARIA relationship attributes need to be implemented.

2. The toolbar buttons incorrectly use aria-pressed. The value of this attribute never changes (i.e. knows when it is pressed), and it is not the proper attribute to use. The correct dynamic ARIA state for a disclosure control is aria-expanded.
3. A major benefit of using aria-pressed is that the open/close behaviour is conveyed by assistive tech before the user operates the button, using standardized terms from the operating system and/or assistive tech. Currently the open/close behaviour is only conveyed after pressing the button, using a custom Drupal.announce() message, in terms localized by Drupal. Using aria-expanded correctly means we no longer need Drupal.announce here.

Proposed resolution

1. Remove the aria-pressed attribute from the toolbar buttons which control trays.
2. Use an aria-controls ID reference for the button/tray relationship. This attribute should be on the element which has the button role, and should point to the tray wrapper which gets display:none.
3. Use aria-expanded to convey current state of a given tray. This attribute should be on the same element which has the button role.
4. Remove the Drupal.announce() message about the tray being opened. It's no longer necessary after adding the aria-expanded property to the button.

Scope: this issue is just about the behaviour of the buttons in the top-level toolbar, which control the visibility of the second-level trays. The behaviour of sub-menus under "mange" when the toolbar tray is in vertical alignment has a separate issue: #3093378: Use ARIA disclosure pattern for submenu buttons in vertical toolbar orientation.

Remaining tasks

User interface changes

Improved semantics for assistive technology.
No changes to visual design or behaviour.

API changes

Data model changes

None.

Release notes snippet