Fix component invalidating widgets unnecessarily

[edwloef]

Given this MCVE:

use iced::widget::{button, column, component, sensor, space, text, Renderer};

fn main() -> iced::Result {
    iced::application(|| false, update, view).run()
}

fn update(state: &mut bool, _: ()) {
    *state = true;
}

fn view(state: &bool) -> impl Into<iced::Element<'_, ()>> {
    column![
        button("show").on_press(()),
        state.then(|| component(Component))
    ]
}

struct Component;

impl<'a> component::Component<'a, ()> for Component {
    type State = bool;
    type Event = ();

    fn update(&mut self, state: &mut bool, _: (), _: &Renderer) -> Option<()> {
        *state = true;
        None
    }

    fn view(&self, state: &bool) -> iced::Element<'a, ()> {
        sensor(if *state {
            text("hi").height(20.8).into()
        } else {
            iced::Element::new(space())
        })
        .on_show(|_| ())
        .into()
    }
}

Clicking "show" wouldn't immediately show "hi", rather it would only show on an external event, such as the window resizing, happening. This PR fixes that.

I also just don't see any reason why this case should invalidate the entire widget tree, rather than just the layout.