Added TabIndex property.

And also added `IsTabStop` property to `InputElement`.

src/Avalonia.Input/InputElement.cs

@@ -71,6 +71,12 @@ namespace Avalonia.Input
         public static readonly DirectProperty<InputElement, bool> IsPointerOverProperty =
             AvaloniaProperty.RegisterDirect<InputElement, bool>(nameof(IsPointerOver), o => o.IsPointerOver);
 
+        /// <summary>
+        /// Defines the <see cref="IsTabStop"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsTabStopProperty =
+            KeyboardNavigation.IsTabStopProperty.AddOwner<InputElement>();
+
         /// <summary>
         /// Defines the <see cref="GotFocus"/> event.
         /// </summary>
@@ -99,6 +105,12 @@ namespace Avalonia.Input
                 "KeyUp",
                 RoutingStrategies.Tunnel | RoutingStrategies.Bubble);
 
+        /// <summary>
+        /// Defines the <see cref="TabIndex"/> property.
+        /// </summary>
+        public static readonly StyledProperty<int> TabIndexProperty =
+            KeyboardNavigation.TabIndexProperty.AddOwner<InputElement>();
+
         /// <summary>
         /// Defines the <see cref="TextInput"/> event.
         /// </summary>
@@ -426,6 +438,15 @@ namespace Avalonia.Input
             internal set { SetAndRaise(IsPointerOverProperty, ref _isPointerOver, value); }
         }
 
+        /// <summary>
+        /// Gets or sets a value that indicates whether the control is included in tab navigation.
+        /// </summary>
+        public bool IsTabStop
+        {
+            get => GetValue(IsTabStopProperty);
+            set => SetValue(IsTabStopProperty, value);
+        }
+
         /// <inheritdoc/>
         public bool IsEffectivelyEnabled
         {
@@ -437,6 +458,16 @@ namespace Avalonia.Input
             }
         }
 
+        /// <summary>
+        /// Gets or sets a value that determines the order in which elements receive focus when the
+        /// user navigates through controls by pressing the Tab key.
+        /// </summary>
+        public int TabIndex
+        {
+            get => GetValue(TabIndexProperty);
+            set => SetValue(TabIndexProperty, value);
+        }
+
         public List<KeyBinding> KeyBindings { get; } = new List<KeyBinding>();
 
         /// <summary>

src/Avalonia.Input/KeyboardNavigation.cs

@@ -5,6 +5,14 @@ namespace Avalonia.Input
     /// </summary>
     public static class KeyboardNavigation
     {
+        /// <summary>
+        /// Defines the TabIndex attached property.
+        /// </summary>
+        public static readonly AttachedProperty<int> TabIndexProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, int>(
+                "TabIndex",
+                typeof(KeyboardNavigation));
+
         /// <summary>
         /// Defines the TabNavigation attached property.
         /// </summary>
@@ -42,6 +50,26 @@ namespace Avalonia.Input
                 typeof(KeyboardNavigation), 
                 true);
 
+        /// <summary>
+        /// Gets the <see cref="TabIndexProperty"/> for an element.
+        /// </summary>
+        /// <param name="element">The container.</param>
+        /// <returns>The <see cref="KeyboardNavigationMode"/> for the container.</returns>
+        public static int GetTabIndex(IInputElement element)
+        {
+            return ((IAvaloniaObject)element).GetValue(TabIndexProperty);
+        }
+
+        /// <summary>
+        /// Sets the <see cref="TabIndexProperty"/> for an element.
+        /// </summary>
+        /// <param name="element">The element.</param>
+        /// <param name="value">The tab index.</param>
+        public static void SetTabIndex(IInputElement element, int value)
+        {
+            ((IAvaloniaObject)element).SetValue(TabIndexProperty, value);
+        }
+
         /// <summary>
         /// Gets the <see cref="TabNavigationProperty"/> for a container.
         /// </summary>
@@ -83,7 +111,7 @@ namespace Avalonia.Input
         }
 
         /// <summary>
-        /// Sets the <see cref="IsTabStopProperty"/> for a container.
+        /// Sets the <see cref="IsTabStopProperty"/> for an element.
         /// </summary>
         /// <param name="element">The container.</param>
         /// <param name="value">Value indicating whether the container is a tab stop.</param>
@@ -93,7 +121,7 @@ namespace Avalonia.Input
         }
 
         /// <summary>
-        /// Gets the <see cref="IsTabStopProperty"/> for a container.
+        /// Gets the <see cref="IsTabStopProperty"/> for an element.
         /// </summary>
         /// <param name="element">The container.</param>
         /// <returns>Whether the container is a tab stop.</returns>