Move functionality out of XULDocument::AddElementToDocumentPost: 1) Convert all XUL link elements into HTML link elements which have code to handle when they are added to the DOM. 2) Move handling of the end of a linkset element into nsXULElement's DoneAddingChildren callback. 3) Move document direction reset to where the root element is created. Differential Revision: https://phabricator.services.mozilla.com/D19739