Sample XFce4 panel plugin.

The plugin is build but not installed. Useful for people who want to write
their own panel plugin.

Be sure to have a look at some other plugins as well, for tricks and ideas.
Look at a plugin that is build separately from the panel, e.g. xfce4-mixer, or
one of the plugins at xfce-goodies.berlios.de, to see how to create
a stand-alone plugin.
