Gate Start on crew, pick starting vehicle, loading screen and help menu

#16 and #12 Start is enabled only with crew/passenger (hold Ctrl to force); the chosen starting vehicle is carried through launch; loading-screen logo; help menu; runtime language picker.

StarterNG/Views/MessageBox.cs

@@ -0,0 +1,56 @@
+using System.Threading.Tasks;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+
+namespace StarterNG.Views;
+
+public enum MessageBoxButtons
+{
+    YesNo
+}
+
+public static class MessageBox
+{
+    /// <summary>Simple Yes/No dialog. Returns true for Yes.</summary>
+    public static async Task<bool> Show(Window owner, string message, string title, MessageBoxButtons _)
+    {
+        var win = new Window
+        {
+            Title = title,
+            Width = 420,
+            SizeToContent = SizeToContent.Height,
+            WindowStartupLocation = WindowStartupLocation.CenterOwner,
+            CanResize = false
+        };
+
+        bool yes = false;
+        var yesBtn = new Button { Content = App.Loc["Yes"], MinWidth = 80, Cursor = new Avalonia.Input.Cursor(Avalonia.Input.StandardCursorType.Hand) };
+        yesBtn.Classes.Add("Accent");
+        yesBtn.Click += (_, _) => { yes = true; win.Close(); };
+
+        var noBtn = new Button { Content = App.Loc["No"], MinWidth = 80, Cursor = new Avalonia.Input.Cursor(Avalonia.Input.StandardCursorType.Hand) };
+        noBtn.Classes.Add("Flat");
+        noBtn.Click += (_, _) => win.Close();
+
+        win.Content = new StackPanel
+        {
+            Margin = new Avalonia.Thickness(16),
+            Spacing = 16,
+            Children =
+            {
+                new TextBlock { Text = message, TextWrapping = TextWrapping.Wrap },
+                new StackPanel
+                {
+                    Orientation = Orientation.Horizontal,
+                    HorizontalAlignment = HorizontalAlignment.Right,
+                    Spacing = 8,
+                    Children = { yesBtn, noBtn }
+                }
+            }
+        };
+
+        await win.ShowDialog(owner);
+        return yes;
+    }
+}