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/web/window.rs

@@ -255,8 +255,10 @@ impl Window {
     }
 
     #[inline]
-    pub fn primary_monitor(&self) -> monitor::Handle {
-        monitor::Handle
+    pub fn primary_monitor(&self) -> Option<RootMH> {
+        Some(RootMH {
+            inner: monitor::Handle,
+        })
     }
 
     #[inline]