MDL-45382 atto_link: Missing context when finding anchors
MDL-45253 editor_atto: selectionFilterMatches includes parents
MDL-45253 editor_atto: Prevent selection of all nodes when selection is empty
MDL-45253 editor_atto: Check for button highlights on change event

This will allow a button to be highlighted when a plugin has updated
the content, not only when the selection has changed.
MDL-45253 editor_atto: Async the highlighting of buttons