Refactor Widgets

The widget structure is something I am not happy with. Currently you have to create a new class for every widget type. This should be turned into a single Widget class and handlers of properties, so we dynamically construct the widget. This also allows easy overriding of default behaviour (when you override you have to retype the whole widget).
Especially refresh should get some attention, since all properties of the widget are set on every call the refresh, even though nothing has changed.
Maybe a nice gui tool like a "widget construction kit" would be nice