Support subtitle selection in Edge (and older Chromium) on Windows in SW mode

[Jaifroid]

As mentioned in #455, at least in Windows 10, subtitles cannot be selected in Edge and in older versions of Chromium (possibly Chrome) in Service Worker mode. I suspect this has something to do with video.js perhaps not understanding the format of relative filenames in Windows (it works in Linux and in WSL - Ubuntu subsystem on Windows), or some other failure point around the encoding of filenames. My reasoning concerning filenames is based on the fact that when I experimentally disable JavaScript (and hence video.js), subtitles are selectable but cannot be found when selected (and console log reports that they cannot be found). Further tests are needed with JS-in-the-ZIM disabled to determine the cause of this. In the meantime, subtitles work correctly in jQuery mode. It has yet to be determined if they work in the latest version of Chromium for Windows.

[Jaifroid]

Further info: video.js does not even ask the Service Worker for the subtitles, which would suggest some low-level issue with the formatting of the filenames. Possibly a difference between handling of escaped or unescaped filenames, but that is just a wild guess at the moment.

[Jaifroid]

Need to check if this bug is still current.