new feature: dialog on saved callback

The patch add a callback type DLGCB_SAVED. The callback is then called each time the dialog is saved into database. This feature is interesting for modules that need to save additional information into database.
- The module can then benefit all database scheduling from dialog (none/realtime/delayed/shutdone).
- Data saved by dialog and by the module are synchronous.
- The cell->flags can help the callback to know if data needs to be inserted, updated or deleted.

Hugues