Improve window setting

[hatoo]

Hi, I'm interested in running iced in vst-rs environment.

For this, there are 2 things to do.

1. (Windows only) Initialize a Window with the given parent window and with other settings.
2. Run event loop discontinuously. The current run method will not return until close. This behavior is not undesirable.

This PR handles 1.
To create a window in VST, we must initialize winit like below.

let wb = winit::window::WindowBuilder::new()
            .with_title("A fantastic window!")
            .with_decorations(false)
            .with_resizable(false)
            .with_parent_window(parent)
            .with_inner_size(LogicalSize {
                width: 400.0,
                height: 300.0,
            });

To do the same thing in iced, we need support setting options with_decorations and with_parent_window.
This is what is the PR does.

For with_decorations, it's just straightforward to implement because it is supported in all platforms in winit.
For with_parent_window it's not straightforward since with_parent_window is Windows-specific API.
So I introduced a simple platform-specific module to implement.

[hecrj]

Thanks for contributing!

I think we can leave the API of the iced crate completely unaffected by this, at least for now.

For the second case you mention, the use case in #50 is definitely related. We should try to come up with a solution for both. I believe there should be a way to split iced_winit functionality for advanced needs on native platforms, while keeping iced completely unaffected.

I'd also like to remind you (and others looking to contribute) that I'd love to know about your use cases before we settle on a specific design! As the contributing guide states, we have a Zulip server and you can also find me on Discord.

[hecrj]

I would only expose PlatformSpecific settings in iced_winit for the time being.

[hatoo]

Sure.
I made iced's platform-specific settings use iced_winit's PlatformSettings.

[hecrj]

Thanks for addressing the feedback! 🎉

I think there is still a detail left to fix, then we can merge this!

[hecrj]

I think this should not be necessary now.

[hatoo]

Oops, I forgot to remove it.
I've removed now.

[hecrj]

The idea is to simply not expose this for now and make advanced users use iced_winit directly instead.

[hatoo]

Make sense!
I've removed extended settings from iced's setting.

[hecrj]

Then we'll simply use iced_winit::settings::PlatformSpecific::default() here.

[hecrj]

Thanks for addressing my feedback!

I have changed a couple of details. Let me know if it's okay and we can merge this!

[hatoo]

Thanks for your work!
It's OK!
Please merge this!