[dev] Horde Notification

[Sebastian Birnbach]

I came across the Horde_Notification lib as a means to display user
messages. The library seems to go way beyond this but I could not find
example code or a writeup on the notification mechanism.

questions:
* what listeners are there in a standard Horde installation?
* what are the supported status levels? horde.error, horde.warning,
horde.message - anything else?
* how can notifications be used for logging?
* can notifications be used for sending emails? The API docs speak of this
possibility but I am not sure how to do it - and why to prefer it over
using the mail application API directly.

Thanks and a good new year to everybody!

  Sebastian