Add option to prevent application shutdown

This update allows the application shutdown to be cancelled. It implements the shouldQuit function, which can prevent the application from quitting if it returns false. Additional checks have been added to ensure cleanup and quit processes are performed only if required. The darwin delegate and linux and windows applications were modified to include this new functionality.

v3/examples/window/main.go

@@ -21,7 +21,7 @@ func main() {
 		Description: "A demo of the WebviewWindow API",
 		Assets:      application.AlphaAssets,
 		Mac: application.MacOptions{
-			ApplicationShouldTerminateAfterLastWindowClosed: false,
+			ApplicationShouldTerminateAfterLastWindowClosed: true,
 		},
 	})
 	app.On(events.Mac.ApplicationDidFinishLaunching, func(event *application.Event) {
@@ -40,8 +40,11 @@ func main() {
 
 	// Create a custom menu
 	menu := app.NewMenu()
-	menu.AddRole(application.AppMenu)
-
+	if runtime.GOOS == "darwin" {
+		menu.AddRole(application.AppMenu)
+	} else {
+		menu.AddRole(application.FileMenu)
+	}
 	windowCounter := 1
 
 	// Let's make a "Demo" menu