StoryTitle related changes

* Improved speed of HTML importing.
* Now, only the StoryTitle passage's text content will be used for the
window title, instead of its raw code.
* Now, if a story has no StoryTitle, or the StoryTitle has no text
content, it will default to the name of the .tws file used to build it,
instead of just "Untitled Story".
* Added a sanity check in case a script overrides
History.prototype.display() but doesn't pass enough arguments.