3488663: Improve Accessibility of readmore widget.

Eric Volkernickrequested to merge
issue/ckeditor_readmore-3488663:3488663-improve-accessibility-of into 3.x
  • Add aria-controls attribute to the readmore toggler
  • Add aria-expanded attribute to the ckeditor-readmore toggler
  • When the readmore widget is opened, shift focus to the readmore wrapper
  • When the readmore widget is closed, keep focus on the readmore toggle

Closes #3488663

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