Added proper saving with subtitle conversion.

FileList no longer notifies SubtitleEditor about changes via DataModel. It checks if there is any
SubtitleEditor opened and requests an update of its stored data. Only if there's no SubtitleEditor
it updates DataModel directly. This is to avoid overwriting data on saving to file (e.g.
SubtitleEditor could have unstaged subtitle text changes that could be overwritten by changing SPF).