monitor: refactor MonitorHandle to store dyn object

This also alters `VideoMode` to be a regular object and not reference
the `MonitorHandle`, since it's a static data.

Given that `VideoMode` set may change during runtime keeping the
reference as a some sort of validity may not be idea and propagating
errors when changing video mode could be more reliable.

src/platform_impl/apple/appkit/mod.rs

@@ -24,4 +24,3 @@ pub(crate) use self::window::Window;
 pub(crate) use self::window_delegate::PlatformSpecificWindowAttributes;
 pub(crate) use crate::cursor::OnlyCursorImageSource as PlatformCustomCursorSource;
 pub(crate) use crate::icon::NoIcon as PlatformIcon;
-pub(crate) use crate::platform_impl::Fullscreen;