Added basic subtitle structures and enums. Refs #112 @30m
Added subtitle selection API. Refs #112 @30m

1. Added SubtitleStream and corresponding FFSubtitleStream.
2. Exposure of subtitle streams from Media.
3. Added subtitle functions from libavcodec.
Fetches basic data from bitmap subtitle. Refs #112 @1.5h
Read subtitle into BufferedImage. Refs #112 @2hours
Implemented Subtitle classes and handler. Refs #112 @30m
Added bitmap subtitle rendering. Refs #112 @2hours
Added text subtitle rendering. No clue if it works. Refs #112 @30mins