Moved the core part from Horde_Notification into different

Moved the core part from Horde_Notification into different
Horde_Notification_Handler classes. Added unit testing.

The core part of the functionality went into
Horde_Notification_Handler_Base. Only the segments that require access
to Horde global state have been split off into separate decorators.

The Horde_Notification_Handler_Alarm decorator delegates alarms to the
alarm system provided in the constructor. It needs access to
Horde_Auth::getAuth().

The Horde_Notification_Handler_Hordelog decorator logs error events
via Horde::logMessage().

As the logging part is useful but should be independant of Horde
global state if possible the Horde_Notification_Handler_Logged
decorator has been added. It allows providing a logger in the
constructor.