Evolutions for 2.1.0

Change log : 
 - Rewrite of request about unread message : handle an infinite number of unread email + the code should be faster to run and use less bandwidth
 - Fix search period of calendar events
 - Display the error to the user : add a warning overlay + the error message is display in the popup/overlay (the UI can be improved)
 - Notification can be displayed longer...
 - Merge changed about locale...

TODO:
 Calendar events rewrite