Apply `Scroll` to widgets immediately when received, rather than during `Ui::set_widgets`

This will probably make reasoning about scrolling a lot easier. This will also make it much easier to handle #602.

`UiEvent::Scroll` would still be delivered to widgets, but would represent the amount the widget was scrolled, rather than an amount of scroll that is required to be handled. Widgets can still opt-in to scrolling regularly by overriding the default `Widget::scroll_kids` method.
