Implement functionality for <param-ref>

For occurrances of <param-ref name="[struct-ref-name]">, the XSLT will find and apply the corresponding <struct-ref> with the name [struct-ref-name], and add it as a parameter. The javascript filter mechanism must be adapted because this introduces a new level of nested collapsible elements.

• <param-ref> is only processed inside of <program> and <struct-ref> tags, because it is an alternative to the <param> tag.

• Using the attribute as-param="[custom name]" we can specify the name of the parameter independently from the <struct-ref>s name (which is only used to identify the corresponding <struct-ref> definition).