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.

src/platform_impl/windows/monitor.rs

@@ -131,8 +131,9 @@ impl Window {
         available_monitors()
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
-        primary_monitor()
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
+        let monitor = primary_monitor();
+        Some(RootMonitorHandle { inner: monitor })
     }
 }