A glue layer between clutter and box2d that provides a special group where the
actors can be set to be static or dynamic in regard to a physics simulation. The
source tree currently contains an embedded version of box2d trunk.

Features:
  - Children of the ClutterBox2d group can be simulated as static or dynamic
    rigid bodies. Collision detection.
  - Specifying the gravity of the world.
  - play/pause control over the box2d physics engine
  - Setting linear and angular velocities of simulated children.
  - automatic position synchronization between clutter and box2d.
  - joints
      - prismatic.
      - revolute.
      - distance (spring)
      - mouse (for dragging objects around and manipulating them).