Extend API for activities notifications
Extend API for activities notifications

Add a new method, called notify_user, to the base
Activity class.

This method can be used by activity developers to
send notifications to the new notification front
end. ie.,

    self.notify_user('New High Score!',
                     'Your score is over 8999')

manuq/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,tchx84/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,tchx84/debian-pkg-sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,puneetgkaur/sugar-toolkit-gtk3,puneetgkaur/backup_sugar_sugartoolkit,quozl/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,tchx84/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,manuq/sugar-toolkit-gtk3,tchx84/sugar-toolkit-gtk3,manuq/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3