Move dialog classes to own file

osu.Game/Screens/Menu/ConfirmExitDialog.cs

@@ -0,0 +1,34 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Graphics.Sprites;
+using osu.Game.Overlays.Dialog;
+
+namespace osu.Game.Screens.Menu
+{
+    public class ConfirmExitDialog : PopupDialog
+    {
+        public ConfirmExitDialog(Action confirm, Action cancel)
+        {
+            HeaderText = "Are you sure you want to exit?";
+            BodyText = "Last chance to back out.";
+
+            Icon = FontAwesome.Solid.ExclamationTriangle;
+
+            Buttons = new PopupDialogButton[]
+            {
+                new PopupDialogOkButton
+                {
+                    Text = @"Goodbye",
+                    Action = confirm
+                },
+                new PopupDialogCancelButton
+                {
+                    Text = @"Just a little more",
+                    Action = cancel
+                },
+            };
+        }
+    }
+}