Segmentation Editor: Add a state indicator ("Loading" and "Rendering") so that it is clear when there is a background operation in progress

The 3D Segmentator Viewer/Editor runs a number of very-long running operations, such as labelmap feature extraction. Though these operations run in the background via web-workers, the 3D viewport does not initialize until they are complete. This can make the application seem like it is stuck or hanging.

For the 3D viewport, there should be an indicator that provides a "Loading" and "Rendering" state indicator which indicates what the current progress is.