use HasRawWindowHandle instead of RawWindowHandle

[BillyDM]

Baseview requires a struct that implements HasRawWindowHandle, which means the user must convert the RawWindowHandle given in PluginUI::ui_open() into a struct that implements that. This PR fixes that by converting it at the baseview level.

It is called TrustedWindowHandle because of it mirrors this: https://github.com/rust-windowing/raw-window-handle/blob/master/src/lib.rs#L210. I don't believe that this is in crates.io yet, so I created a temporary one.

Instead of the user doing this:

fn ui_open(parent: RawWindowHandle) -> WindowOpenResult<Self::Handle> {
    let settings = iced_baseview::Settings {
        window: WindowOpenOptions {
            title: String::from("iced-baseplug-examples gain"),
            size: Size::new(Self::ui_size().0 as f64, Self::ui_size().1 as f64),
            scale: WindowScalePolicy::SystemScaleFactor,
        },
        flags: (),
    };

    // TODO: Fix this mess in baseplug.
    struct ParentWindow(RawWindowHandle);
    unsafe impl HasRawWindowHandle for ParentWindow {
        fn raw_window_handle(&self) -> RawWindowHandle {
            self.0
        }
    }

    let handle =
        iced_baseview::IcedWindow::<ui::GainUI>::open_parented(&ParentWindow(parent), settings);

    Ok(handle)
}

They can now do this:

fn ui_open(parent: &impl HasRawWindowHandle) -> WindowOpenResult<Self::Handle> {
    let settings = iced_baseview::Settings {
        window: WindowOpenOptions {
            title: String::from("iced-baseplug-examples gain"),
            size: Size::new(Self::ui_size().0 as f64, Self::ui_size().1 as f64),
            scale: WindowScalePolicy::SystemScaleFactor,
        },
        flags: (),
    };

    let handle =
        iced_baseview::IcedWindow::<ui::GainUI>::open_parented(parent, settings);

    Ok(handle)
}