Retrieve Blender 'Collision Bounds' to set Urho CollisionShape

Now we can select the CollisionShape to apply.
The shape setting in the Export panel is used when 'Merge objects' is true or when 'Collision Bounds' is off (located in Properties > Physics). Otherwise 'Collision Bounds' shape is retrieved.
Note that physicsSettings is a list currently only handling one item (the shape). This allows to later add support for other physics settings like mass, mask, restitution...