Improve Widget trait

[PoignardAzur]

I want to make some pretty deep changes to the Widget trait. The ultimate trait would look something like this:

pub trait Widget2: Any {
    fn on_event(&mut self, ctx: &mut EventCtx, event: &Event);

    fn lifecycle(&mut self, ctx: &mut LifeCycleCtx, event: &LifeCycle);

    fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size;
    fn compute_max_intrinsic(&mut self, axis: Axis, ctx: &mut LayoutCx, bc: &BoxConstraints) -> f64;

    fn accessibility(&mut self, cx: &mut AccessCx);

    // Maybe some other arguments after the switch to Vello
    fn paint(&mut self, ctx: &mut PaintCtx);

    fn children(&self) -> SmallVec<[WidgetRef<'_, dyn Widget>; 16]>;

    // Got to figure out what argument the callback takes
    fn call_on_children(&mut self, callback: impl FnMut(&mut impl Widget));

    fn get_debug_text(&self) -> Option<String>;

    // --- Auto-generated implementations ---

    fn make_trace_span(&self) -> Span;

    fn get_child_at_pos(&self, pos: Point) -> Option<WidgetRef<'_, dyn Widget>>;
    fn get_child_with_id(&self, id: WidgetId) -> Option<WidgetRef<'_, dyn Widget>>;

    fn call_on_child_at_pos(&mut self, pos: Point, callback: impl FnOnce(&mut impl Widget));
    fn call_on_child_with_id(&mut self, id: WidgetId, callback: impl FnOnce(&mut impl Widget));

    fn type_name(&self) -> &'static str;
    fn short_type_name(&self) -> &'static str;
}

Notable changes:

• Accessibility support is baked in. I'm not sure this is the right way though.
• We add compute_max_intrinsic method to layout pass.
• We add call_on_children method. That method should replace the boilerplate code in container widgets that propagates events. I'm not sure how to make it work type-system-wise.
• Generally speaking, we give up on the idea of Widget being completely dyn-safe. This mean we'll need an ErasedWidget type, same idea as in Panoramix.