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/linux/wayland/event_loop.rs

@@ -648,8 +648,9 @@ impl<T> EventLoopWindowTarget<T> {
         available_monitors(&self.outputs)
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
-        primary_monitor(&self.outputs)
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
+        // Wayland doesn't have a notion of primary monitor.
+        None
     }
 }
 
@@ -1121,19 +1122,6 @@ impl MonitorHandle {
     }
 }
 
-pub fn primary_monitor(outputs: &OutputMgr) -> MonitorHandle {
-    outputs.with_all(|list| {
-        if let Some(&(_, ref proxy, _)) = list.first() {
-            MonitorHandle {
-                proxy: proxy.clone(),
-                mgr: outputs.clone(),
-            }
-        } else {
-            panic!("No monitor is available.")
-        }
-    })
-}
-
 pub fn available_monitors(outputs: &OutputMgr) -> VecDeque<MonitorHandle> {
     outputs.with_all(|list| {
         list.iter()

src/platform_impl/linux/wayland/window.rs

@@ -10,8 +10,7 @@ use crate::{
     error::{ExternalError, NotSupportedError, OsError as RootOsError},
     monitor::MonitorHandle as RootMonitorHandle,
     platform_impl::{
-        platform::wayland::event_loop::{available_monitors, primary_monitor},
-        MonitorHandle as PlatformMonitorHandle,
+        platform::wayland::event_loop::available_monitors, MonitorHandle as PlatformMonitorHandle,
         PlatformSpecificWindowBuilderAttributes as PlAttributes,
     },
     window::{CursorIcon, Fullscreen, WindowAttributes},
@@ -410,8 +409,9 @@ impl Window {
         available_monitors(&self.outputs)
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
-        primary_monitor(&self.outputs)
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
+        // Wayland doesn't have a notion of primary monitor.
+        None
     }
 
     pub fn raw_window_handle(&self) -> WaylandHandle {