Add Window::set_window_level API

This adds `Window::set_window_level` to control the preferred
z level of the window.

Co-authored-by: Markus Siglreithmaier <m.siglreith@gmail.com>
Co-authored-by: Kirill Chibisov <contact@kchibisov.com>
Co-authored-by: Mads Marquart <mads@marquart.dk>

src/platform_impl/macos/appkit/window.rs

@@ -296,6 +296,8 @@ pub struct NSWindowLevel(pub NSInteger);
 
 #[allow(dead_code)]
 impl NSWindowLevel {
+    #[doc(alias = "BelowNormalWindowLevel")]
+    pub const BELOW_NORMAL: Self = Self((kCGNormalWindowLevel - 1) as _);
     #[doc(alias = "NSNormalWindowLevel")]
     pub const Normal: Self = Self(kCGNormalWindowLevel as _);
     #[doc(alias = "NSFloatingWindowLevel")]