Media Information handling. [WIP]

Media Information retrieval and handling. 

This is still work in Progress. Still need to get the MediaStreamInfo structure and accessors in place. Also need to refactor tag  handling methods. 
This patch is a rework based on the review comments on the pull request I last made: https://github.com/sdroege/gst-player/pull/5 . I haven't closed that one yet.

I thought this will give you an idea as to what I am doing, and can let me know if I'm doing anything wrong (or right). Progress has been slow because of work commitments. (really sorry about that).
g_clear_pointer() is NULL-safe