Fix mantis issue 3969: Notification icon on taskbar could NOT disappear unless restart DataSpaceSync client
Fix mantis issue 4030: NO upload/download icon show in file sync transmission notifications
Fix mantis issue 4033: Credential check was NOT auto performed when first time go to Credentials Settings tab
Fix mantis issue 4046: Circular progress bar may cause GUI hang (crash) in Mac
Fix mantis issue 3786: Notifications should be implemented with NotifyOSD
Improved notification toggle of config file to have an effect on linux notifications

(cherry picked from commit c6fce2ef6a2e4c8d92b682e8ee9a00f14dc1ea4d)