fixes: Renamed StartupScreen to StartupScreenIndex

samples/ControlCatalog/MainWindow.xaml.cs

@@ -19,7 +19,7 @@ namespace ControlCatalog
             this.InitializeComponent();
             this.AttachDevTools(new Avalonia.Diagnostics.DevToolsOptions()
             {
-                StartupScreen = 1,
+                StartupScreenIndex = 1,
             });
             //Renderer.DrawFps = true;
             //Renderer.DrawDirtyRects = Renderer.DrawFps = true;

src/Avalonia.Diagnostics/Diagnostics/DevToolsOptions.cs

@@ -24,8 +24,8 @@ namespace Avalonia.Diagnostics
         public Size Size { get; set; } = new Size(1280, 720);
 
         /// <summary>
-        /// Get or set the startup screen number where the DevTools window will be displayed.
+        /// Get or set the startup screen index where the DevTools window will be displayed.
         /// </summary>
-        public int? StartupScreen { get; set; }
+        public int? StartupScreenIndex { get; set; }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs

@@ -218,7 +218,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void SetOptions(DevToolsOptions options)
         {
-            StartupScreenIndex = options.StartupScreen;
+            StartupScreenIndex = options.StartupScreenIndex;
         }
     }
 }