ui: Initial better user error reporting (#1503)

This update the "No keys" dialog and block starting NSP/XCI/NCA without firmware.

Also propose to the user if they want to install firmware if they start an untrimmed XCI and remove KEYS.md as it was completely outdated.

PS: Also fix a bug with "&" in URL with OpenUrl on Windows.

Ryujinx/Ui/Diagnostic/GuideDialog.cs

@@ -0,0 +1,36 @@
+using Gtk;
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Text;
+
+namespace Ryujinx.Ui.Diagnostic
+{
+    internal class GuideDialog : MessageDialog
+    {
+        internal static bool _isExitDialogOpen = false;
+
+        public GuideDialog(string title, string mainText, string secondaryText) : base(null, DialogFlags.Modal, MessageType.Other, ButtonsType.None, null)
+        {
+            Title = title;
+            Icon = new Gdk.Pixbuf(Assembly.GetExecutingAssembly(), "Ryujinx.Ui.assets.Icon.png");
+            Text = mainText;
+            SecondaryText = secondaryText;
+            WindowPosition = WindowPosition.Center;
+            Response += GtkDialog_Response;
+
+            Button guideButton = new Button();
+            guideButton.Label = "Open the Setup Guide";
+
+            ContentArea.Add(guideButton);
+
+            SetSizeRequest(100, 10);
+            ShowAll();
+        }
+
+        private void GtkDialog_Response(object sender, ResponseArgs args)
+        {
+            Dispose();
+        }
+    }
+}