[GREENFOOT-66] Progress indicator needed for export

Export can take a rather long time. There should be a progress (or busy) indicator somewhere.

Maybe a non-modal dialogue with a twiddling (indefinite) progress bar, that has buttons "Open Web Page" and "Close" ("Open Web Page" is initially disabled, and enables when finished).

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Issue metadata

• Issue type: Task
• Priority: Medium
• Fix versions: 1.4.6