Make 'current_monitor' return 'Option<MonitorHandle>'

On certain platforms window couldn't be on any monitor
resulting in failures of 'current_monitor' function.

Such issue was happening on Wayland, since the window
isn't on any monitor, unless the user has drawn something into it.

Returning 'Option<MonitorHandle>' will give an ability to
handle such situations gracefully by properly indicating that
there's no current monitor.

Fixes #793.

src/platform_impl/windows/window.rs

@@ -577,10 +577,10 @@ impl Window {
     }
 
     #[inline]
-    pub fn current_monitor(&self) -> RootMonitorHandle {
-        RootMonitorHandle {
+    pub fn current_monitor(&self) -> Option<RootMonitorHandle> {
+        Some(RootMonitorHandle {
             inner: monitor::current_monitor(self.window.0),
-        }
+        })
     }
 
     #[inline]