Dropdown accessibility label is not updated when combo value changes

[f1ames]

Type of report

Bug

Provide detailed reproduction steps (if any)

Our richcombo plugin (and all other plugins which use it underneath) does not comply with the accessibility recommendation described in https://www.w3.org/WAI/WCAG21/Understanding/label-in-name.html - Ensure that elements labelled through their content must have their visible text as part of their accessible name.

This is caused by the fact that richcombo accessibility label is not updated when combo value changes so it always holds the initial lable.

1. Go to https://codepen.io/f1ames/pen/rNMdgYL?editors=1010 or any editor instance with Font plugin.
2. Open browser dev console and find combo label element <span id="cke_15_label" class="cke_combo_label">Font</span> (ids may vary).
3. Change Font dropdown value.

Expected result

Combo label element text changes and contains name of the value selected.

Actual result

Combo label element text does not change and doesn't reflect selected value.

Other details

• Browser: All (Tested on Chrome)
• OS: All (Tested on Linux)
• CKEditor version: 4.10.0+ (but it seems it always worked like that)
• Installed CKEditor plugins: richcombo

[CKEditorBot]

Closed in #4580