Implemented EXT-6783(normal sub task) - Implement saving of unread notifications.

Utilized old save and load notification code.
Main concern was with notifications that have complex responder - UserGiveItem, ObjectGiveItem. Those responders are object with own fields that need to persist through sessions.

Notifications that should be saved are marked with persist="true" in notifications.xml
Notifications using functor responders are saved automatically.
Notifications using object responders need additional tuning. Responder object should be a) serializable(implement LLNotificationResponderInterface), b) registered with LLResponderRegistry.

At this point following notifications persist through sessions: UserGiveItem, ObjectGiveItem, TeleportOffered, FrienshipOffered.

Reviewed by Mike Antipov - https://codereview.productengine.com/secondlife/r/211/

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branch : notifications