Rework 'Fullscreen::Borderless' enum variant

This changes 'Fullscreen::Borderless' enum variant from
'Fullscreen::Borderless(MonitorHandle)' to
'Fullscreen::Borderless(Option<MonitorHandle>)'. Providing
'None' to it will result in picking the current monitor.

src/platform_impl/macos/window_delegate.rs

@@ -450,7 +450,7 @@ extern "C" fn window_will_enter_fullscreen(this: &Object, _: Sel, _: id) {
                 // Otherwise, we must've reached fullscreen by the user clicking
                 // on the green fullscreen button. Update state!
                 None => {
-                    let current_monitor = window.current_monitor_inner();
+                    let current_monitor = Some(window.current_monitor_inner());
                     shared_state.fullscreen = Some(Fullscreen::Borderless(current_monitor))
                 }
             }