MacOS minimize support

[chrisduerr]

This has been reported to Alacritty in alacritty/alacritty#2534.

Currently winit exposes an API on all platforms to set the window visibility using the set_visible method. If this is called on macOS, the window is hidden without adding it to the dock, this is working as intended.

However, on macOS a different method of hiding windows exists too, this is called miniaturize in their docs which makes the window invisible and adds it to the dock. I believe this is what happens when the minimize button is clicked in the window decorations, but as far as I can tell there's no way to call this explicitly from winit.

The generated cocoa-rs bindings provide this method on NsWindow. So I think it should be relatively trivial to implement this using the WindowsExtMacOS, since I don't think it's available in this fashion on any other platform.

Alternatively, this could be declared as out of scope, so downstream consumers would have to use the ns_window call instead to do this themselves.

[felixrabe]

I think that's perfectly in scope. I just looked at https://docs.rs/winit/0.20.0-alpha1/winit/window/struct.Window.html because I wondered whether this could not be handled by some platform-independent set_minimized method. Turns out, there isn't one.

[felixrabe]

Related: #783 (Minimizing window on Windows causes WindowEvent::Resized(size=0) event).
Related: #208 (Need to get info about window state)

[felixrabe]

Opened #985 (Add Window::set_minimized method) about the missing method.

[kchibisov]

I guess this bug should be closed now.