OPS_swagger - add notifications to fetch operation (pending, failed, succeeded)

[jmartasek]

At the moment when the URL for the swagger file is changed the change will not take effect unless the fetch button is pressed and the fetch operation completed successfully.

To fix the UX I suggest:

• Perform the fetch operation on URL change automatically
• Make the fetch operation a blocking call with dialogue showing the progress and ability to cancel it
• Only allow to save the node changes if the fetch operation succeeded.