Media info and tags support.

This patch adds support to retrieve media information for
audio/video/text streams.
Tags from each individual stream for Audio. Video and Text are parsed and stored in the appropriate structure (GstPlayerAudioStreamInfo, GstPlayerVideoStreamInfo, and GstPlayerTextStreamInfo respectively).

The primary structure is GstPlayerMediaInfo. An instance of this structure for each stream type is in GstPlayer. 

Once the data is parsed from each of these streams, data is passed to the client via the signals (AUDIO_DATA_UPDATED, VIDEO_DATA_UPDATED, TEXT_DATA_UPDATED).