Added some helper methods for constraints and btMatrix3x3,

Thanks to  francois, See Issue 466

Prevent crash in SoftBodyDemo, when picking constraints and demo-mode switches to next demo.

Demos/OpenGL/DemoApplication.h

@@ -55,6 +55,8 @@ protected:
 	///constraint for mouse picking
 	btTypedConstraint*		m_pickConstraint;
 
+	virtual void removePickingConstraint();
+
 	btCollisionShape*	m_shootBoxShape;
 
 	float	m_cameraDistance;