Application::run returns on native platforms

[AndiHofi]

Added an on_exit() -> Option<Box<dyn FnOnce>> function to the iced::Application trait that provides that callback.

When the returned closure does not actually exit the application then iced::Application::run() can return as well, allowing the program to continue when the main window is closed.

For iced_web it could make sense to invoke the on_exit() closure when unloading the page, did not try to implement that though.

The main use-case for this is to allow to change the exit code of the application.

[hecrj]

Maybe I'm missing something, but if we are leveraging run_return and allowing Application::run to return control, then the on_exit method seems redundant.

In other words, users can just write:

let _ = Application::run()?;

on_exit();

[AndiHofi]

yeah - I somehow overlooked that. I did not want to change the control flow of Application::run() it currently only returns when initializing the application fails.

I realized that the run() method may return normally when running through the code for the pull-request.

Letting it return in all cases would be simpler than any callback - unless returning from Application::run does not work with all backends.

[AndiHofi]

Removed the on_exit() callback entirely and added an example that Application::run returns. Also that it may be called in a loop.

Added another commit that demonstrates the same when calling iced_winit::run or iced_glutin::run

[hecrj]

Can we keep the changes to the the bare minimum (i.e. just the run_return)?

We can think about additional examples later. If we start adding examples for every change, they will be very hard to navigate 😅

[AndiHofi]

sure, I get rid of the examples...

and rename the pull request. There is no more on_exit callback

[hecrj]

Looks good!

I made some minor changes, but it should be ready to go! 🎉