Make 'primary_monitor' return 'Option<MonitorHandle>'

Certain platforms like Wayland don't have a concept of
primary Monitor in particular. To indicate that
'primary_monitor' will return 'None' as well as in cases
where the primary monitor can't be detected.

Fixes #1683.

examples/video_modes.rs

@@ -3,7 +3,13 @@ use winit::event_loop::EventLoop;
 fn main() {
     simple_logger::init().unwrap();
     let event_loop = EventLoop::new();
-    let monitor = event_loop.primary_monitor();
+    let monitor = match event_loop.primary_monitor() {
+        Some(monitor) => monitor,
+        None => {
+            println!("No primary monitor detected.");
+            return;
+        }
+    };
 
     println!("Listing available video modes:");