StoryMenu tweaks and fixes

* Now, the "no Start passage" warning is suppressed if a StoryIncludes
passage is present.
* Now, if no Start passage is present, it is created when you choose New
Passage.
* Gave the #storyTitle, #storySubtitle, #storyAuthor and #storyMenu
elements in all targets the class "storyElement", as another CSS hook.
* Altered the CSS of the targets so that Twine links always have
cursor:pointer regardless of DOM location.
* <<back>> now works in a .storyElements passage.
* <<choice>> now produces an error message if it's used in a
.storyElements passage.