NXP-12077: Map icon color to sync states: enabled, transferring, disabled, stopping
NXP-12077: Display 'Quitting...' in the menu when quitting
NXP-12063: Don't send proxy test request if no proxy is configured
NXP-12077: Use gif icons with transparent background
NXP-12077: Finally use png icons with transparent background as gif ones don't appear in systray under OS X
Merge branch 'master' into 5.6.0