Fix mesh name changing 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.
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