Removed some unnecessary/non-existent methods from MediaStreamGStream implementation
Even more cleanup + imlemented some new MediaStreamPlayer methods

- Moved some MediaPlayerBackendBase's variables that are not used
  in MediaStreamPlayerBackend to DefaultMediaPlayerBackend
- Added a vitual method "pipeline()" to MediaPlayerBackendBase to
  abstract the underlying player's gstreamer pipeline. In
  DefaultMediaPlayerBackend it is a "playbin" element and in
  MediaStreamPlayerBackend it is a GstPipeline.

Signed-off-by: Nick Diego Yamane <nick.yamane@openbossa.org>