Provide a way to clean up content and settings

I am still of the opinion that it is entirely feasible a site admin might want to uninstall CCK Gallery and keep the content and ImageCache settings. I'm nervous about providing a hook_uninstall() because one slip and someone could, even in the future, accidentally invoke it and destroy their data.

I propose an alternative - a "delete all CCK Gallery content and settings" button in admin that you must press *prior* to removing CCK Gallery if you don't want the content any more. Avoids leaving dangerous and undocumented "uninstall" options lying around on the modules page.