Debugger: Improvements to hrev47421.

- Isolate the filesystem query/result list building into a separate
  worker thread in order to prevent blocking the window thread in
  case the query winds up being a bit more time consuming. This
  doesn't yet handle intelligent prefetching (and associated can of
  synchronization worms), but that will come once time permits.
- Also fixes a missing break statement introduced in the aforementioned
  commit, though that one shouldn't have caused any actual harm.