[3.3.x] network stream panel: improve legacy subtitle lookup feature

The network stream panel includes an optional legacy subtitles lookup feature that is doing a brute-force search of all known and supported subtitles file extension on http servers.

This MR moves that search to the background thread and replaces a busy loop by proper thread sleeping.

This fixes a noticeable number of terminations of the app because the main thread was often locked for more than 48s.

Backport of !695 (merged)