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/event_loop.rs

@@ -34,6 +34,7 @@ use crate::{
     dpi::{PhysicalPosition, PhysicalSize},
     event::{DeviceEvent, Event, Force, KeyboardInput, Touch, TouchPhase, WindowEvent},
     event_loop::{ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootELW},
+    monitor::MonitorHandle as RootMonitorHandle,
     platform_impl::platform::{
         dark_mode::try_dark_mode,
         dpi::{become_dpi_aware, dpi_to_scale_factor, enable_non_client_dpi_scaling},
@@ -255,8 +256,9 @@ impl<T> EventLoopWindowTarget<T> {
         monitor::available_monitors()
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
-        monitor::primary_monitor()
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
+        let monitor = monitor::primary_monitor();
+        Some(RootMonitorHandle { inner: monitor })
     }
 }