Add Color Change to Tray Icon During Synchronization

First of all: Thank you very much for the fork! 

It would be nice to have the tray icon change color when it is "synchronizing" to make it more obvious. Currently the icon is "only" animated, which could be difficult to see. 

Alternatively, add an option for a gray tray icon, that changes to the current (white and blue colored) icon when it is syncing. 