Consider offering an option to invoke timer callback on drop

[mweber15]

I have a cleanup timer that should have its callback invoked either when the interval elapses or when the timer is dropped. In my case the drop coincides with the program exiting, so I can accomplish this manually. It would be nice if there was a tidier way -- maybe I am missing something that already exists.

fn main() -> Result<(), slint::PlatformError> {
    let main_window = MainWindow::new()?;
    let clear_timer = std::rc::Rc::new(slint::Timer::default());
    let clear_timer_ref = clear_timer.clone();
    main_window.on_accepted(move || {
        if let Some(window) = weak.upgrade() {
            clear_timer_ref.start(
                slint::TimerMode::SingleShot,
                std::time::Duration::from_secs(10),
                || { clear(); },
            );
        }
    });
    // ...
    let result = main_window.run();
    if clear_timer.running() {
        clear();
    }
    result
}