[3.0] qt: allow opening links from module's help text

Primarily useful for out-of-tree plugins, in case they want to link to plugin's home page, bug tracker, donation page, etc.

refs #27798 (closed), !3203 (merged)

Backport of a feature doing only minimal changes: faf8b85a caf14331

---

Tried to keep changes to a minimal in this backport.

I don't think the changes in !3203 (merged) break the library ABI (correct me if I'm wrong), so I can make a more truthful/"fuller" backport if this minimal attempt is deemed unsatisfactory.