Implementation of the 'widget' passage tag

Widgets are a feature introduced by Thomas M. Edwards
(http://www.motoslave.net/sugarcube/). They are passages which can be
<<display>>ed using a concise syntax ("<<passagename>>") and can take
parameters as if they were proper macros (which in this implementation
are accessed via the parameter() function). This commit implements
widgets as a passage tag rather than, as in their original formulation,
a <<widget>> macro. It also adds IDE colour-coding for widget passages.
Added syntax colouration for widget invocations

Widget invocations are now a brown colour to distinguish them from
magenta macro invocations.