Pointer attribute get/set functions for particles.
Added collision shape pointer attribute for particles. This will change
soon, for now is the easiest way to keep track of things.
Removed unused variables.
Particle validate function for adding particle rigid bodies.
Fix potential issue from early exit when no constraints exist.
Cleanup: Renamed a number of internal rigidbody functions to clarify
the procedure:

  world_build:  create rbRigidBody instances for stuff in Scene
  validate_*:   ensure a rbRigidBody exists for an Object/particle/etc.
  sync_*:       define Bullet settings for an Object/particle/etc. based
                on Blender Scene data
  world_apply:  copy back data from Bullet after a step to the Blender
                Scene data
Cleanup: Separated object and constraint build loops into own functions.
Renamed some more API methods to follow the new naming scheme.
Added inline comment about possible memory management optimization.