Fix wrongly resetting mesh name everytime we change visibility

When load a 2dm file twice then change the visibility of the second mesh in CMB, the mesh name would change everytime correspondingly. It's because we would set the collectionName to original_mesh first and assign a unique name to mesh everytime (since the current two meshes would have same name after the reset). An if statement is added so that we only assign a unique mesh name when it's empty.

Fix smtk issue #10 (closed).