Transformation relationships are now established in ConnectToTrackerStep.onExit() instead of in PlaceSpatialFiducialsStep.onEntry()
Modifications:
  * Relative path is used to load sounds
  * Now it is possible to add data to the scene in the LoadSceneStep. This is in order to avoid duplicated information
  * Added to the USGuidedProcedure:
      - crosshairEnable()
      - crosshairDisable()
    Those methods are not used because it is not clear how and where to change from one to the other.
  * Stylus model is added to the scene if the transformation is valid.
  * A widget was added to the main widget to indicate if the tools are visible or not.