Created a root exception class for all other exceptions within LW
Added a 'renderer' class method for all widgets and an associated content-renderer method which should not require most widgets now to have to implement their own 'content_renderer' class. However, this means that the renderer now should no longer rely on 'opts' in the constructor but get everything directly from the widget. This will in fact lead to better separation of controller (widget) and view (renderer)
The widget creation through the PluginManager was a bit confused on what is a plugin and what is a widget. This should clean it up. Resulted in a method name change
Added a RedirectWidget exception which allows one widget to redirect the handling of a request to a new widget by throwing this exception. See the 'Topology Editor' plugin for an example where the slice create widget kicks it over to a slice monitor widget when a new slice is requested