decodebin:

  A bin with a sinkpad that decodes the data into raw formats. It works by sending

  the input data through a typefind element and then recursively autoplugs elements 

  from the registry until a raw format is obtained. It will then create a new ghostpad

  on itself to signal the app of the new pad. 

  Decodebin will also remove pads when they are removed from the stream.

  TODO

   - reuse of decoderbin, cleanup in READY state

   - threading after demuxing?

   - new_media events should be handled.

   - caching of elements.

   - abstract more elements, pads (typefind, ...);

   The autoplugging happens as follows:

   1) typefind is added internally to the bin.

   2) the have_type signal is connected to typefind.

   3) in the have_type callback the close_pad_link function is called

   4) close_pad_link checks the type on the pad, if it is raw, a ghostpad

      is created and autoplugging for that pad stops.

   5) if the type of the pad is not raw, a list of possible elements that

      can connect to this type is generated in find_compatibles.

   6) try_to_link_1 with the element list is called. The function will loop

      over the element list and will try to connect one of the elements to

      the pad. If the link works, a call is made to close_link.

   7) close_link loops over all the source pads of the element and 

      recursively calls 4) for any ALWAYS pad. For elements with

      a SOMETIMES pad, a structure is set up and is passed to the callback

      of the new_pad signal.

   8) in the new_pad callback, 4) is called to try to autoplug the

      new pad.

playbasebin:

  A bin with an uri property. It will find the right source element from the registry

  and connect a decoderbin to it. When going to the PAUSED state, it will iterate the

  decoderbin and listen for new pad signals from it. It will connect a queue to each

  new pad and will iterate the decoderbin until one of the queues is filled. It is

  assumed that by that time all the streams will be found so that when leaving the

  PAUSED state, one can query the number of streams in the media file with the given

  uri.

  Playbasebin internally groups related streams together in a GstPlayBaseGroup. This

  is particulary important for chained oggs. Initially, a new group is created in 

  the 'building' state. All new streams will be added to the building group until

  no-more-pads is signaled or one of the preroll queues overflows. When this happens,

  the group is commited to a list of groups ready for playback. PlaybaseBin will then

  attach a padprobe to each stream to figure out when it finished. It will remove

  the current group and install the next playable group, then.

  Before going to the PLAYING state, it is possible to connect a custom element to

  each of the streams. To do that, you have to add the element to the bin and then

  connect the pad(s) from the stream(s). You do not have to add the elements in

  a thread, the bin will take care of then when it's needed. You are allowed to use

  threads inside the elements, of course.

  The bin tries to be smart and doesn't add a queue when there is only one possible 

  stream.

  TODO

  - reuse, cleanup in ready state

  - when the first pad is closed, it's possible that another dynamic element is

    added somewhere so that we need a queue for the first pad as well.

playbin:

   Extends playbasebin, sets up default audiosink and videosink for first audio/video

   stream detected. implements seeking and querying on the configured sinks.

   It also waits for new notifications from playbasebin about any new groups that are

   becomming active. It then disconnects the sinks and reconnects them to the new

   pads in the group.

   TODO

   - reuse, refcounting, cleanup in READY state

   - be smarter about replugging the sinks instead of removing them and readding them.

   - Do not crap out when the audio device is in use.

general

   TODO

   - playlist support. maybe use a playlist bin that streams the contents of the

     playlist on a pad, interleaved with new_media events. Also add a tuner 

     interface while we're at it.