Passage editor helpers

* Now, when you close a passage that contains broken links, Twine offers
to create the new passages that each link refers to.
* Now, when you close a passage that contains the [img] syntax with an
external image, Twine offers to import the image into the story. If it
does so, it replaces the URL in the passage text with the imported
passage's name.
* Added "Test Play From Here" to the passage editor's Passage menu.