The system tray widget is divided into three parts: core, ui and protocols.

Core
----
Core provides the three basic concepts that drive the widget:
    * Task: an icon in the tray
    * Notification: a visual notification from an application
    * Job: a long running job such as a file transfer

Protocol implements a source for Tasks, Notifications and/or Jobs and has a set
of signals used to announce the coming and going of its items. At its discretion,
a Protocol can create (or destroy) Tasks, Notifications and Jobs at any time.

A Manager class is provided in Core that looks after and tracks the three kinds of items
along with all the Protocols. The Manager instantiates the Protocols and relays the
critical signals, allowing the widget to simply pay attention to the Manager and
treat all Tasks, Notifications and Jobs as if they came from the same place. The Manager
is able to treat all Protocols generically using the Protocol API.

All Task implementations provide a way to return a QGraphicsWidet for given host.
The mapping between hosts and widgets is handled automatically by the Task class, and
this, along with the central Manager, allows any Task to be shared simultaneously across
multiple widgets.

Notifications and Jobs don't have this problem as they simply provide access to
data which the ui is responsible for showing in some way.

Also provided in the core is a Task specifically for managing the Extenders of a host
Plasma::Applet for Jobs and Notifications call, rather creatively, ExtenderTask.


Protocols
---------
The following protocols currently exist:

DBusSystemTrayProtocol: support for the dbus driven NotificationItem; it provides Tasks;
                        for information on the design of this protocol, consult the
                        KStatusNotifierItem documentation

FdoProtocol: support for the XEmbed based freedesktop.org system tray specification;
             it provides Tasks, though these tasks do not work properly in more than one
             widget at a time due to limitations in the XEmbed system

JobsProtocol: access to the applicationjobs DataEngine; it provides Jobs

NotificationsProtocol: access to the notificadtions DataEngine; it provides Notifications

PlasmoidProtocol: features Plasmoids-wrapped-in-Tasks; is not currently fully functional


The UI
------
The user interface consists of a Plasma::Applet class, a layout class and a TasksArea which
contains the Tasks published. The UI features the ability to order as well as hide items,
register new Tasks, etc.