feat: media track improvements

[huynhsontung]

• Support .idx and .sub files
• Use proper neighbouring files query to find subtitles in the current directory.
• Improve external subtitle handling, including auto subtitle load on playback. Fixes #190
• Unify video, audio, and subtitle track to have the same base class MediaTrack
• Parse language tag to display language name whenever possible. Fixes #235
• Show a tool tip if the subtitle name is too long