[IPlayer] Add struct SPlayerSubtitleStreamInfo and GetSubtitleStreamInfo(int iStream, SPlayerSubtitleStreamInfo &info).

[IPlayer/GUI/LegacyPlayer/JSON] Remove GetSubtitleName(int iStream, CStdString &strStreamName) and replace the calls to it with calls to GetSubtitleStreamInfo(int iStream, SPlayerSubtitleStreamInfo &info).
[AMLPlayer] !!!Needs Review, imo strange logic!!!
Remove GetSubtitleName(int iStream, CStdString &strStreamName) and move its functionality to GetSubtitleStreamInfo(int iStream, SPlayerSubtitleStreamInfo &info).