Make sure we're visiting an id when we cache it

This might want some further rethinking at some point, anyway it works.

lisp/gnorb-org.el: Function `gnorb-org-add-id-hash-entry' will sometimes
		   need to visit the proper heading first.

lisp/gnorb-utils.el: Function `gnorb-trigger-todo-action' needs to tell
		     `gnorb-org-add-id-hash-entry where to go.