Use qFuzzyIsNull instead of comparing to 0 with qFuzzyCompare
Optimized and fixed joint properties

* Simplified and inlined getters when the properties are never modified
  by Box2D
* Added missing scaling in DistanceJoint.length getter
* Fixed FrictionJoint.maxTorque setting the MaxForce when the joint is
  already created
* Check FrictionJoint.maxForce and FrictionJoint.maxTorque for validity
* When the PulleyJoint is not yet created, return the starting lengths
  from getCurrentLengthA and getCurrentLengthB instead of 0
* Check GearJoint.ratio for validity
* Fixed MotorJoint.angularOffset to be properly inverted
* Fixed change notification signals of MouseJoint properties
* Fixed conversion of PrismaticJoint.motorSpeed
* Fixed conversion of WelfJoint.referenceAngle in getter
Introduced toBox2DBody inline convenience function

Makes code more readable.