Variables in image syntax + other fixes

* It should now be possible to supply variables to the image syntax, as
an image passage name - for instance [img[$pictureName]].
* Shorthand <<display>>: variable default was causing $vars provided as
arguments to mess up horridly.  This should fix this.
* Made a number of macro error messages a little better - displaying "$"
instead of "state.history[0].variables." when referring to the macro
invocation.
* StorySettings: fixed the Obfuscate checkbox using "on" instead of
"swap".