Jeff/couzteau fix 6356 stale image update

Optimized version of @couzteau's implementation (#6443).  I tested this on 1750 and 1647 and it works pretty good.  I'm not sure why after merging master into this branch that it's showing more than it should change wise.

fix #6356 - forcing CEF to reload a cached image when its timestamp on disk changes.

This pull does two things: 
a) it enables file sync manager to detect if an image file viewed by a custom viewer changes
b) it enables a work around for CEFs failure to reload files that changed on disk.

Ideally none of this code would not be necessary if CEF automatically purged its cache of files loaded via the file protocol based on their timestamp on disk.