Inside the library, multiple content items can be bookmarked now. Additional properties have been added to _Editor_properties.ts_ for this:

Bookmark_button_add_multi_text: "Add Bookmarks",
  Bookmark_button_add_multi_tooltip: "Bookmark the selected content items",
  Bookmark_button_remove_multi_tooltip: "Remove bookmarks",
  Bookmark_button_remove_multi_text: "Remove Bookmarks",

(CMS-21454)