Messages
--------

Messages are refcounted lightweight objects to signal the application
of pipeline events. 

Messages are implemented as a subclass of GstData with a generic 
GstStructure as the content. This allows for writing custom messages without
requiring an API change while allowing a wide range of different types
of messages.

Messages are posted by objects in the pipeline and are passed to the 
application using the GstBus. 


Message types
-------------

  GST_MESSAGE_EOS:

    The pipeline went to EOS. This means that all the sink elements in the
    pipeline posted the EOS message to the bus.
  
  GST_MESSAGE_ERROR:
  
    An element in the pipeline got into an error state. The message carries
    a GError and a debug string describing the error. This usually means that
    part of the pipeline is not streaming anymore.
  
  GST_MESSAGE_WARNING:
  
    An element in the pipeline encountered a condition that made it produce a
    warning. This could be a recoverable decoding error or some other non fatal
    event. The pipeline continues streaming after a warning.
  
  GST_MESSAGE_INFO:

    An element produced an informational message.
  
  GST_MESSAGE_TAG:

    An element decoded metadata about the stream. The message carries a GstTagList
    with the tag information.
  
  GST_MESSAGE_BUFFERING:
  
    An element is buffering data and that could potentially take some time.
  
  GST_MESSAGE_STATE_CHANGED:

    An element changed state in the pipeline. The message carries the old an new
    state of the element.
  
  GST_MESSAGE_STEP_DONE:

    An element stepping frames has finished.
  
  GST_MESSAGE_NEW_CLOCK:

    A new clock was selected for the pipeline.

  GST_MESSAGE_STRUCTURE_CHANGE:

    The pipeline changed of structure, This means elements were added or removed or
    pads were linked or unlinked.

  GST_MESSAGE_STREAM_STATUS:

   An element posted information about the stream it is handling. This could include
   information about the length of the stream.
    
  GST_MESSAGE_APPLICATION:

    An element posted an application specific message.