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/windows/mod.rs

@@ -11,7 +11,6 @@ pub(crate) use crate::cursor::OnlyCursorImageSource as PlatformCustomCursorSourc
 use crate::event::DeviceId;
 use crate::icon::Icon;
 use crate::platform::windows::{BackdropType, Color, CornerPreference};
-use crate::platform_impl::Fullscreen;
 
 #[derive(Clone, Debug, PartialEq)]
 pub struct PlatformSpecificWindowAttributes {