Can't clear the list of applications that use a widget in the admin UI

[Anthony Lenton]

The 'Applications' field is required in the admin UI form for creating/editing an ApplicationWidget. This has two similar and unwanted side-effects:

 - You can't create an app-less widget. If you want to create a widget without apps you need to create it with an app and then remove the widget from the Application's edit UI.

 - You can't remove all apps from an existing widget. If you want to do this, you need to do this by editing each app that uses the widget, to remove the widget from the app (or remove all apps from the widget except one, and then remove the widget from that last app).