Registers a :class:`~horizon.Panel` with this dashboard.

def register(cls, panel):
        """Registers a :class:`~horizon.Panel` with this dashboard."""
        panel_class = Horizon.register_panel(cls, panel)
        # Support template loading from panel template directories.
        panel_mod = import_module(panel.__module__)
        panel_dir = os.path.dirname(panel_mod.__file__)
        template_dir = os.path.join(panel_dir, "templates")
        if os.path.exists(template_dir):
            key = os.path.join(cls.slug, panel.slug)
            loaders.panel_template_dirs[key] = template_dir
        return panel_class