Jalview unresponsive when starting up if local network connection is poor

One result of JAL-594 was rethreading of the various background tasks that Jalview launches on start up, and many of these were moved to the 'prompt' thread - which allows modal dialogs from different background threads to be correctly sequenced to avoid GUI thread deadlocks (JAL-1111).

Some of the new services seem to be using the modal dialog method on the Jalview.Desktop to perform actions involving data retrieval - this is unreliable since the thread blocks all other GUI actions.