Expose the collection behavior on mac (always on top hides in mission control)

[jkelleyrtp]

Expose the nswindowbehavior managed on macos.

Also, would be nice if "set_always_on_top" also set this flag so the window wouldn't disappear from mission control when you make it sticky. I would expect the behavior for "always on top" to match that of sticky notes on mac, which both float on top and participate in window management.

More info here:
https://developer.apple.com/documentation/appkit/nswindowcollectionbehavior/nswindowcollectionbehaviormanaged

Here's the code:

        // https://developer.apple.com/documentation/appkit/nswindowcollectionbehavior/nswindowcollectionbehaviormanaged
        #[cfg(target_os = "macos")]
        {
            use cocoa::appkit::NSWindowCollectionBehavior;
            use cocoa::base::id;
            use objc::{msg_send, sel, sel_impl};
            use tao::platform::macos::WindowExtMacOS;

            unsafe {
                let window: id = window.ns_window() as id;
                let _: () = msg_send![window, setCollectionBehavior: NSWindowCollectionBehavior::NSWindowCollectionBehaviorManaged];
            }
        }