Fixes and optimizations

* Removed unnecessary check in Box2DBody::at_fixture
* Inlined all getters for properties that are never changed dynamically
  by Box2D to return the value from the b2BodyDef.
* Fixed the value of the 'awake' property, which was always returning
  the value in the b2BodyDef because of a missing 'return' keyword
  for the case where the body was created.
* Changed type of linearDamping, angularDamping and angularVelocity to
  'float' to match the Box2D variable they are stored in.
* Fixed Box2DBody::angularVelocity to return the dynamic value when
  appropriate, which might have changed from the b2BodyDef value.
* Fixed Box2DBody::setAngularVelocity to not compare the degree variable
  with the radian variable.
* Don't do a qFuzzyCompare in Box2DBody::setGravityScale. There is no
  reason why the gravity scale shouldn't match exactly.
* Fixed conversion of mass.
* Removed unnecessary emit functions.
* Don't have DebugDraw darken the view automatically, for performance reasons.
* Removed the need to keep a list of bodies in Box2DWorld