Fix Listview for qt style

[6he]

This is a follow up to PR #9696.

As suggested a struct property is used to save the reference values.
This also fixes the scrolling bug for the Qt style (Fixes #9208).

[6he]

I tried to write a test as suggested in #9696, but I could not trigger the bug in the test.
In the test the ListView items are only updated after the PointerReleased event is send (see below).
Is there another way to trigger an update of the ListView items without releasing the mousebutton?

import { ListView } from "std-widgets.slint";

export component TestCase inherits Window {
    width: 100px;
    height: 100px;

    property<[length]> item-heights: [
         50px,
        150px,
        250px,
        350px,
        450px,
    ];

    list := ListView {
        width: parent.width;
        height: parent.height;

        mouse-drag-pan-enabled: true;
        vertical-scrollbar-policy: always-on;

        for height in item-heights: Rectangle {
            height: height;
        }
    }

    out property<length> viewport-height <=> list.viewport-height;
    out property<length> viewport-y <=> list.viewport-y;
}

/*

``````rust
use slint::{LogicalPosition, platform::WindowEvent};

let instance = TestCase::new().unwrap();

let button = slint::platform::PointerEventButton::Left;
let position = LogicalPosition::new(50.0, 50.0);

// move mouse to trigger instantiation of ListView items
instance.window().dispatch_event(WindowEvent::PointerMoved{ position });

println!("----> viewport-height: {}", instance.get_viewport_height());
println!("----> viewport-y: {}", instance.get_viewport_y());

// Scroll vertically by dragging the ListView
let position = LogicalPosition::new(50.0, 70.0);
instance.window().dispatch_event(WindowEvent::PointerMoved{ position });
instance.window().dispatch_event(WindowEvent::PointerPressed{ position, button });
let position = LogicalPosition::new(50.0, 19.0);
instance.window().dispatch_event(WindowEvent::PointerMoved{ position });

// the first item should be discarded at this point because it is no longer visible,
// but the items (and the estimated viewport-height) are only updated when the mousebutton is released.

instance.window().dispatch_event(WindowEvent::PointerReleased{ position, button });

println!("----> viewport-height 2: {}", instance.get_viewport_height());
println!("----> viewport-y 2: {}", instance.get_viewport_y());

[ogoffart]

Thanks for the follow up.

[ogoffart]

The use of one single assignment rather than 4 would be slightly more efficient:
self.saved-values = { pressed-value: -root.value, maximum: ..... }

[6he]

I adjusted the first commit.

[ogoffart]

Thank you!