Re-write DownloadManager, DownloadThread(Worker) using
a custom thread pool implementation for speed. We don't
want to create a new thread for each url when the same
thread can be re-used.
Re-designed options frame
Changed "highest available" video format to "default"
Fixed password bug
Fixed "Download subtitle file by languages" bug
Removed Options
Rate Limit
Added Options
Embed Subtitles to mp4
Video Password (Vimeo etc..)
Added version check for settings file