feat(header_bar): Show window menu on right click

With this, apps like cosmic-term require no changes to show the
compositor window menu when the header is right clicked.

src/app/cosmic.rs

@@ -71,6 +71,7 @@ pub enum Message {
     SurfaceClosed(window::Id),
     /// Activate the application
     Activate(String),
+    ShowWindowMenu,
 }
 
 #[derive(Default)]
@@ -412,6 +413,10 @@ impl<T: Application> Cosmic<T> {
                     return self.app.update(msg);
                 }
             }
+            Message::ShowWindowMenu => {
+                #[cfg(not(feature = "wayland"))]
+                return window::show_window_menu(window::Id::MAIN);
+            }
         }
 
         iced::Command::none()