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

[mitchmindtree]

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.

[mitchmindtree]

Closing in favour of #711.