Notification Center

Finished Tasks:
 - Notification center is now working perfectly.
 - Adding 3 fields for each notification, friend icon image,
notification title and notification.
 - Adding notifications to saved properties so each new notification is
appended to the old notifications.
 - Max number of notifications is set to 25, and the oldest one is
deleted for more.

Remaining Tasks:
 - Adding some new features in notification center like clicking on the
notification or changes made by friend