Use aria disclosure pattern in vertical toolbar submenu toggle buttons.

Closes #3093378

added 2622 commits

• d16b3970...6439ca10 - 2620 commits from branch project:11.x
• f8d5c0ec - Merge branch '11.x' into 3093378-use-aria-disclosure
• 923dbe60 - Check aria-expanded attribute in the Nightwatch test

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added 2 commits

• 69720142 - Make sure changes do not affect stable9 theme
• 23f9f666 - Remove a redundant CSS rule

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added 39 commits

• 23f9f666...e8e94812 - 36 commits from branch project:11.x
• 91b4e56d - Merge branch '11.x' into 3093378-use-aria-disclosure
• 263a9044 - Revert "Remove a redundant CSS rule"
• fce0db2e - Use text (inner HTML) instead of aria-labelledby

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added 1 commit

• f6e64c20 - Adjust byte count in the performance test

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resolved all threads

added 35 commits

• f6e64c20...15860ab3 - 32 commits from branch project:11.x
• 59deefac - Reapply "Remove a redundant CSS rule"
• c40b5fde - Use .visually-hidden instead of text-indent: -9999px
• 64c73706 - Merge branch '11.x' into 3093378-use-aria-disclosure

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added 23 commits

• 64c73706...27629c7a - 21 commits from branch project:11.x
• d4d8ae63 - Merge branch '11.x' into 3093378-use-aria-disclosure
• ac974297 - Assert the button text before and after toggling it

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added 1 commit

• 13fd5094 - The targeted element is Configuration, not Structure

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added 1 commit

• 5ec5ba4b - Use getAccessibleName() in test

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