Remove magic nrs from link/joint property 'loadData(..)' methods

[gavanderhoorn]

Example: LinkGeometryProperty::loadData() (here):

        else if (geometry_->type == urdf::Geometry::MESH)
        {
          [..]
          if (name == "Scale")
          {
            //Remaining sub-properties are for Mesh
            QList<QtProperty *> sub_items =  top_item_ ->subProperties()[2]->subProperties();
            [..]
          }
        }

The assumption here is that mesh sub properties are always at index 3, which is not under this applications control, and may thus change at any time.

At a minimum, the magic number (2) should be removed and a named constant used. Ideally -- if possible -- the sub properties of top_item_ should be searched for the Mesh properties, and that index should be used.

[gavanderhoorn]

Related: #79.