Suggestions for improvements

[lassoan]

This is a follow-up from review of the extension for extensions manager submission:

• Register a sample data set that downloads a concrete lung CT and use that in the tutorial
• Add wait cursor when starting the processing
• Instead of adding "It can take some minutes" to the Apply button, add a label and show some status updates there. You can start out displaying "It can take some minutes", but provide updates as the processing goes.
• Processing fails for standard CTChest data set. The user needs a meaningful explanation why the processing has failed and what to do about it.