Interface proposed by the mediacontrol module:

The mediacontrol module holds the PlayerFactory class, which will
create a new player instance.

In the lib/advene/player directory are located multiple modules
player-dependent, that each define a Player class. The PlayerFactory
is responsible for importing the right module and instanciating a
Player() from it.

* PlayerFactory

It defines a 
create_player()
method which returns a Player instance.

* Player

It implements the various methods of the MediaControl IDL. Note that
this API is expected to change in future version: it proved useful,
but is too verbose and uncommon.

It defines the following methods:
- is_active(): Checks whether the player is active.
- stop_player(): Stops the player instance
- restart_player(): Restarts the player
- check_player(mediafile): Checks that the player is active. If not,
                           starts it and load mediafile in the
			   playlist
- update_status(status, position): Updates the status of the player.
  This is the upper level method, which takes care of status caching
  and parameter conversion (long to VLC.Position)

- position_update(): Updates the current status information.
  After its call, the following attributes will be updated:
  - status : player status
  - stream_duration : duration of the stream (in ms)
  - current_position_value : current position (in ms)

* Used MediaControl IDL methods

player.display_text
player.exit
player.get_default_media
player.playlist_add_item
player.playlist_get_list
player.snapshot
player.sound_get_volume
player.sound_set_volume
player.stop

* Used player methods

player.init
player.is_active
player.position_update
player.restart_player
player.update_status

* Used player attributes

player.status
player.current_position_value
player.stream_duration
player.relative_position

The IDL constants (PositionOrigin and PositionKey) should be available
as attributes of the Player class so that advene modules can use them
without having to import other modules (such as VLC from
MediaControl.so).
