* Introduce new widget helper function to evaluate variables.Functions are evaluated as parameterised filter strings, macros as text with textual substitution of parameters and variables, and procedures and widgets as plain text
* Refactor the function operator and unknown operator to use the new helper
* Use the new helper to evaluate variables within filter strings, thus fixing a bug whereby functions called in such a way were being returned as plain text instead of being evaluated
* Refactor the transclude widget to use the new helper
* Update tests
