Backwards Compat: org-agenda-bulk-unmark-all requires txt property

A few of the marking/unmarking functions in org-agenda (for bulk
operations) require the 'txt text property to differentiate between
different search results.

org-ql-view.el

@@ -974,6 +974,7 @@ return an empty string."
            ;; FIXME: Use proper prefix
            (concat "  " it)
            (org-add-props it properties
+             'txt string
              'org-agenda-type 'search
              'org-category category
              'todo-state todo-keyword