Add some sample output to the render method, move the constants API stuff out of the main debug bar panel class.
Introduce and instantiate the new WW_Toggle_Constants API class separately from the Debug Bar Panel class for Toggle.
Notable changes to the moved API code include changing the _constants property to public, and introduction of several new methods for handling adding and removing constants, as well as a new action, 'ww_toggle_constants_init' for plugins to hook into.