Make the names of indexed symbols clickable

[pkgw]

Right now, when a named module appears in a WEB code listing, you can click on that module to pull up some information about it. This is done using "inserts" of the kind StartModuleReference, which causes the TeX-emitting code to insert a special macro which then expands to a <a> link special that invokes a callback implemented in the app.

We should support the same kind of functionality for indexed symbols, which appear very frequently in WEB listings. Most of the implementation would be parallel to the above. The extra part would be figuring out where to log a new "symbol ref" insert in the pretty-printing code.

Clicking on such a symbol could potentially bring up an info page, the right part of the symbol index, or maybe just start "paging" the places where that symbol appears. The latter would be most effective if the pager can actually indicate what it's paging and highlight hits to the paged item (#4).