Stale image preview - refresh image  when the timestamp of image file changes on disk

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.