structure view saved in project file as zero length entry - project load doesn't open viewer correctly

An in-development bug reported for posterity, in case it resurfaces:

Noticed during 2.11.2 testing that Jmol views were not being imported correctly after saving to a project file. Instead of the original view, a red error message was displayed, complaining of 'invalid format' for the input session, followed by a 'load APPEND' message with the viewed files.

This turned out to be due to the viewer's zip entry in the project having zero length.. possibly due to a race condition where the StructureViewer.saveSession() routine returned a file before the viewer had finished saving its to the file (as noted by [~gmungoc] as a TODO in the development code).

Fix: add a sleep/wait loop watching the file that ends when its length no longer changes. The fix is rough and ready, and may well need tuning in future releases.